APPENDIX.

Drawn by your kindness, I have come here to give these lectures, and now that my visit to America hasbecome almost a thing of the past, I look back upon it as a memory without a single stain. No lecturer was ever rewarded as I have been. From this vantage-ground, however, let me remind you that the work of the lecturer is not the highest work; that in science, the lecturer is usually the distributor of intellectual wealth amassed by better men. And though lecturing and teaching, in moderation, will in general promote their moral health, it is not solely or even chiefly, as lecturers, but as investigators, that your highest men ought to be employed. You have scientific genius amongst you—not sown broadcast, believe me, it is sown thus nowhere—but still scattered here and there. Take all unnecessary impediments out of its way. Keep your sympathetic eye upon the originator of knowledge. Give him the freedom necessary for his researches, not overloading him, either with the duties of tuition or of administration, nor demanding from him so-called practical results—above all things, avoiding that question which ignorance so often addresses to genius: 'What is the use of your work?' Let him make truth his object, however unpractical for the time being it may appear. If you cast your bread thus upon the waters, be assured it will return to you, though it be after many days.

Mr. William Spottiswoode introduced some years ago to the members of the Royal Institution, in a very striking form, a series of experiments on the spectra of polarized light. With his large Nicol prisms he in the first place repeated and explained the experiments of Foucault and Fizeau, and subsequently enriched the subject by very beautiful additions of his own. I here append a portion of the abstract of his discourse:—

'It is well known that if a plate of selenite sufficiently thin be placed between two Nicol's prisms, or, more technically speaking, between a polarizer and analyzer, colour will be produced. And the question proposed is, What is the nature of that colour? is it simply a pure colour of the spectrum, or is it a compound, and if so, what are its component parts? The answer given by the wave theory is in brief this: In its passage through the selenite plate the rays have been so separated in the direction of their vibrations and in the velocity of their transmission, that, when re-compounded by means of the analyzer, they have in some instances neutralized one another. If this be the case, the fact ought to be visible when the beam emerging from the analyzer is dispersed by the prism; for then we have the rays of all the different colours ranged side by side, and, if any be wanting, theirabsence will be shown by the appearance of a dark band in their place in the spectrum. But not only so; the spectrum ought also to give an account of the other phenomena exhibited by the selenite when the analyzer is turned round, viz. that when the angle of turning amounts to 45°, all trace of colour disappears; and also that when the angle amounts to 90°, colour reappears, not, however, the original colour, but one complementary to it.'You see in the spectrum of the reddish light produced by the selenite a broad but dark band in the blue; when the analyzer is turned round the band becomes less and less dark, until when the angle of turning amounts to 45° it has entirely disappeared. At this stage each part of the spectrum has its own proportional intensity, and the whole produces the colourless image seen without the spectroscope. Lastly, as the turning of the analyzer is continued, a dark band appears in the red, the part of the spectrum complementary to that occupied by the first band; and the darkness is most complete when the turning amounts to 90°. Thus we have from the spectroscope a complete account of what has taken place to produce the original colour and its changes.'It is further well known that the colour produced by a selenite, or other crystal plate, is dependent upon the thickness of the plate. And, in fact, if a series of plates be taken, giving different colours, their spectra are found to show bands arranged in different positions. The thinner plates show bands in the parts of the spectrum nearest to the violet, where the waves are shorter, and consequently give rise to redder colours; while the thicker show bands nearer to the red, where the waves are longer and consequently supply bluer tints.'When the thickness of the plate is continually increased, so that the colour produced has gone through the complete cycle of the spectrum, a further increase of thickness causes a reproduction of the colours in the same order; but it will be noticed that at each recurrence of the cycle the tintsbecome paler, until when a number of cycles have been performed, and the thickness of the plate is considerable, all trace of colour is lost. Let us now take a series of plates, the first two of which, as you see, give colours; with the others which are successively of greater thickness the tints are so feeble that they can scarcely be distinguished. The spectrum of the first shows a single band; that of the second, two; showing that the second series of tints is not identical with the first, but that it is produced by the extinction of two colours from the components of white light. The spectra of the others show series of bands more and more numerous in proportion to the thickness of the plate, an array which may be increased indefinitely. The total light, then, of which the spectrum is deprived by the thicker plates is taken from a greater number of its parts; or, in other words, the light which still remains is distributed more and more evenly over the spectrum; and in the same proportion the sum total of it approaches more and more nearly to white light.'These experiments were made more than thirty years ago by the French philosophers, MM. Foucault and Fizeau.'If instead of selenite, Iceland spar, or other ordinary crystals, we use plates of quartz cut perpendicularly to the axis, and turn the analyzer round as before, the light, instead of exhibiting only one colour and its complementary with an intermediate stage in which colour is absent, changes continuously in tint; and the order of the colour depends partly upon the direction in which the analyzer is turned, and partly upon the character of the crystal,i.e.whether it is right-handed or left-handed. If we examine the spectrum in this case we find that the dark band never disappears, but marches from one end of the spectrum to another, orvice versâ, precisely in such a direction as to give rise to the tints seen by direct projection.'The kind of polarization effected by the quartz plates is called circular, while that effected by the other class ofcrystals is called plane, on account of the form of the vibrations executed by the molecules of æther; and this leads us to examine a little more closely the nature of the polarization of different parts of these spectra of polarized light.'Now, two things are clear: first, that if the light be plane-polarized—that is, if all the vibrations throughout the entire ray are rectilinear and in one plane—they must in all their bearings have reference to a particular direction in space, so that they will be differently affected by different positions of the analyzer. Secondly, that if the vibrations be circular, they will be affected in precisely the same way (whatever that may be) in all positions of the analyzer. This statement merely recapitulates a fundamental point in polarization. In fact, plane-polarized light is alternately transmitted and extinguished by the analyzer as it is turned through 90°; while circularly polarized light [if we could get a single ray] remains to all appearance unchanged. And if we examine carefully the spectrum of light which has passed through a selenite, or other ordinary crystal, we shall find that, commencing with two consecutive bands in position, the parts occupied by the bands and those midway between them are plane-polarized, for they become alternately dark and bright; while the intermediate parts,i.e.the parts at one-fourth of the distance from one band to the next, remain permanently bright. These are, in fact, circularly polarized. But it would be incorrect to conclude from this experiment alone that such is really the case, because the same appearance would be seen if those parts were unpolarized,i.e.in the condition of ordinary lights. And on such a supposition we should conclude with equal justice that the parts on either side of the parts last mentioned (e.g. the parts separated by eighth parts of the interval between two bands) were partially polarized. But there is an instrument of very simple construction, called a "quarter-undulation plate," a plate usually of mica, whose thickness is an odd multiple of a quarter of a wave-length, which enables us to discriminate between light unpolarizedand circularly polarized. The exact mechanical effect produced upon the ray could hardly be explained in detail within our present limits of time; but suffice it for the present to say that, when placed in a proper position, the plate transforms plane into circular and circular into plane polarization. That being so, the parts which were originally banded ought to remain bright, and those which originally remained bright ought to become banded during the rotation of the analyzer. The general effect to the eye will consequently be a general shifting of the bands through one-fourth of the space which separates each pair.'Circular polarization, like circular motion generally, may of course be of two kinds, which differ only in the direction of the motion. And, in fact, to convert the circular polarization produced by this plate from one of these kinds to the other (say from right-handed to left-handed, orvice versâ), we have only to turn the plate round through 90°. Conversely, right-handed circular polarization will be changed by the plate into plane-polarization in one direction, while left-handed will be changed into plane at right angles to the first. Hence if the plate be turned round through 90° we shall see that the bands are shifted in a direction opposite to that in which they were moved at first. In this therefore we have evidence not only that the polarization immediately on either side of a band is circular; but also that that immediately on the one side is right-handed, while that immediately on the other is left-handed[28].'If time permitted, I might enter still further into detail, and show that the polarization between the plane and the circular is elliptical, and even the positions of the longer and shorter axes and the direction of motion in each case. But sufficient has, perhaps, been said for our present purpose.'Before proceeding to the more varied forms of spectral bands, which I hope presently to bring under your notice, I should like to ask your attention for a few minutes to the peculiar phenomena exhibited when two plates of selenite giving complementary colours are used. The appearance of the spectrum varies with the relative position of the plates. If they are similarly placed—that is, as if they were one plate of crystal—they will behave as a single plate, whose thickness is the sum of the thicknesses of each, and will produce double the number of bands which one alone would give; and when the analyzer is turned, the bands will disappear and re-appear in their complementary positions, as usual in the case of plane-polarization. If one of them be turned round through 45°, a single band will be seen at a particular position in the spectrum. This breaks into two, which recede from one another towards the red and violet ends respectively, or advance towards one another according to the direction in which the analyzer is turned. If the plate be turned through 45° in the opposite direction, the effects will be reversed. The darkness of the bands is, however, not equally complete during their whole passage. Lastly, if one of the plates be turned through 90°, no bands will be seen, and the spectrum will be alternately bright and dark, as if no plates were used, except only that the polarization is itself turned through 90°.'If a wedge-shaped crystal be used, the bands, instead of being straight, will cross the spectrum diagonally, the direction of the diagonal (dexter or sinister) being determined by the position of the thicker end of the wedge. If two similar wedges be used with their thickest ends together, they will act as a wedge whose angle and whose thickness is double of the first. If they be placed in the reverse position they will act as a flat plate, and the bands will again cross the spectrum in straight lines at right angles to its length.'If a concave plate be used the bands will dispose themselves in a fanlike arrangement, their divergence depending upon the distance of the slit from the centre of concavity.'If two quartz wedges, one of which has the optic axis parallel to the edge of the refractory angle, and the other perpendicular to it, but in one of the planes containing the angle (Babinet's Compensator), the appearances of the bands are very various.'The diagonal bands, besides sometimes doubling themselves as with ordinary wedges, sometimes combine so as to form longitudinal (instead of transverse) bands; and sometimes cross one another so as to form a diaper pattern with bright compartments in a dark framework, andvice versâ, according to the position of the plates.'The effects of different dispositions of the interposed crystals might be varied indefinitely; but enough has perhaps been said to show the delicacy of the method of spectrum analysis as applied to the examination of polarized light.'

'You see in the spectrum of the reddish light produced by the selenite a broad but dark band in the blue; when the analyzer is turned round the band becomes less and less dark, until when the angle of turning amounts to 45° it has entirely disappeared. At this stage each part of the spectrum has its own proportional intensity, and the whole produces the colourless image seen without the spectroscope. Lastly, as the turning of the analyzer is continued, a dark band appears in the red, the part of the spectrum complementary to that occupied by the first band; and the darkness is most complete when the turning amounts to 90°. Thus we have from the spectroscope a complete account of what has taken place to produce the original colour and its changes.

'It is further well known that the colour produced by a selenite, or other crystal plate, is dependent upon the thickness of the plate. And, in fact, if a series of plates be taken, giving different colours, their spectra are found to show bands arranged in different positions. The thinner plates show bands in the parts of the spectrum nearest to the violet, where the waves are shorter, and consequently give rise to redder colours; while the thicker show bands nearer to the red, where the waves are longer and consequently supply bluer tints.

'When the thickness of the plate is continually increased, so that the colour produced has gone through the complete cycle of the spectrum, a further increase of thickness causes a reproduction of the colours in the same order; but it will be noticed that at each recurrence of the cycle the tintsbecome paler, until when a number of cycles have been performed, and the thickness of the plate is considerable, all trace of colour is lost. Let us now take a series of plates, the first two of which, as you see, give colours; with the others which are successively of greater thickness the tints are so feeble that they can scarcely be distinguished. The spectrum of the first shows a single band; that of the second, two; showing that the second series of tints is not identical with the first, but that it is produced by the extinction of two colours from the components of white light. The spectra of the others show series of bands more and more numerous in proportion to the thickness of the plate, an array which may be increased indefinitely. The total light, then, of which the spectrum is deprived by the thicker plates is taken from a greater number of its parts; or, in other words, the light which still remains is distributed more and more evenly over the spectrum; and in the same proportion the sum total of it approaches more and more nearly to white light.

'These experiments were made more than thirty years ago by the French philosophers, MM. Foucault and Fizeau.

'If instead of selenite, Iceland spar, or other ordinary crystals, we use plates of quartz cut perpendicularly to the axis, and turn the analyzer round as before, the light, instead of exhibiting only one colour and its complementary with an intermediate stage in which colour is absent, changes continuously in tint; and the order of the colour depends partly upon the direction in which the analyzer is turned, and partly upon the character of the crystal,i.e.whether it is right-handed or left-handed. If we examine the spectrum in this case we find that the dark band never disappears, but marches from one end of the spectrum to another, orvice versâ, precisely in such a direction as to give rise to the tints seen by direct projection.

'The kind of polarization effected by the quartz plates is called circular, while that effected by the other class ofcrystals is called plane, on account of the form of the vibrations executed by the molecules of æther; and this leads us to examine a little more closely the nature of the polarization of different parts of these spectra of polarized light.

'Now, two things are clear: first, that if the light be plane-polarized—that is, if all the vibrations throughout the entire ray are rectilinear and in one plane—they must in all their bearings have reference to a particular direction in space, so that they will be differently affected by different positions of the analyzer. Secondly, that if the vibrations be circular, they will be affected in precisely the same way (whatever that may be) in all positions of the analyzer. This statement merely recapitulates a fundamental point in polarization. In fact, plane-polarized light is alternately transmitted and extinguished by the analyzer as it is turned through 90°; while circularly polarized light [if we could get a single ray] remains to all appearance unchanged. And if we examine carefully the spectrum of light which has passed through a selenite, or other ordinary crystal, we shall find that, commencing with two consecutive bands in position, the parts occupied by the bands and those midway between them are plane-polarized, for they become alternately dark and bright; while the intermediate parts,i.e.the parts at one-fourth of the distance from one band to the next, remain permanently bright. These are, in fact, circularly polarized. But it would be incorrect to conclude from this experiment alone that such is really the case, because the same appearance would be seen if those parts were unpolarized,i.e.in the condition of ordinary lights. And on such a supposition we should conclude with equal justice that the parts on either side of the parts last mentioned (e.g. the parts separated by eighth parts of the interval between two bands) were partially polarized. But there is an instrument of very simple construction, called a "quarter-undulation plate," a plate usually of mica, whose thickness is an odd multiple of a quarter of a wave-length, which enables us to discriminate between light unpolarizedand circularly polarized. The exact mechanical effect produced upon the ray could hardly be explained in detail within our present limits of time; but suffice it for the present to say that, when placed in a proper position, the plate transforms plane into circular and circular into plane polarization. That being so, the parts which were originally banded ought to remain bright, and those which originally remained bright ought to become banded during the rotation of the analyzer. The general effect to the eye will consequently be a general shifting of the bands through one-fourth of the space which separates each pair.

'Circular polarization, like circular motion generally, may of course be of two kinds, which differ only in the direction of the motion. And, in fact, to convert the circular polarization produced by this plate from one of these kinds to the other (say from right-handed to left-handed, orvice versâ), we have only to turn the plate round through 90°. Conversely, right-handed circular polarization will be changed by the plate into plane-polarization in one direction, while left-handed will be changed into plane at right angles to the first. Hence if the plate be turned round through 90° we shall see that the bands are shifted in a direction opposite to that in which they were moved at first. In this therefore we have evidence not only that the polarization immediately on either side of a band is circular; but also that that immediately on the one side is right-handed, while that immediately on the other is left-handed[28].

'If time permitted, I might enter still further into detail, and show that the polarization between the plane and the circular is elliptical, and even the positions of the longer and shorter axes and the direction of motion in each case. But sufficient has, perhaps, been said for our present purpose.

'Before proceeding to the more varied forms of spectral bands, which I hope presently to bring under your notice, I should like to ask your attention for a few minutes to the peculiar phenomena exhibited when two plates of selenite giving complementary colours are used. The appearance of the spectrum varies with the relative position of the plates. If they are similarly placed—that is, as if they were one plate of crystal—they will behave as a single plate, whose thickness is the sum of the thicknesses of each, and will produce double the number of bands which one alone would give; and when the analyzer is turned, the bands will disappear and re-appear in their complementary positions, as usual in the case of plane-polarization. If one of them be turned round through 45°, a single band will be seen at a particular position in the spectrum. This breaks into two, which recede from one another towards the red and violet ends respectively, or advance towards one another according to the direction in which the analyzer is turned. If the plate be turned through 45° in the opposite direction, the effects will be reversed. The darkness of the bands is, however, not equally complete during their whole passage. Lastly, if one of the plates be turned through 90°, no bands will be seen, and the spectrum will be alternately bright and dark, as if no plates were used, except only that the polarization is itself turned through 90°.

'If a wedge-shaped crystal be used, the bands, instead of being straight, will cross the spectrum diagonally, the direction of the diagonal (dexter or sinister) being determined by the position of the thicker end of the wedge. If two similar wedges be used with their thickest ends together, they will act as a wedge whose angle and whose thickness is double of the first. If they be placed in the reverse position they will act as a flat plate, and the bands will again cross the spectrum in straight lines at right angles to its length.

'If a concave plate be used the bands will dispose themselves in a fanlike arrangement, their divergence depending upon the distance of the slit from the centre of concavity.

'If two quartz wedges, one of which has the optic axis parallel to the edge of the refractory angle, and the other perpendicular to it, but in one of the planes containing the angle (Babinet's Compensator), the appearances of the bands are very various.

'The diagonal bands, besides sometimes doubling themselves as with ordinary wedges, sometimes combine so as to form longitudinal (instead of transverse) bands; and sometimes cross one another so as to form a diaper pattern with bright compartments in a dark framework, andvice versâ, according to the position of the plates.

'The effects of different dispositions of the interposed crystals might be varied indefinitely; but enough has perhaps been said to show the delicacy of the method of spectrum analysis as applied to the examination of polarized light.'

The singular and beautiful effect obtained with a circular plate of selenite, thin at the centre, and gradually thickening towards the circumference, is easily connected with a similar effect obtained with Newton's rings. Let a thin slice of light fall upon the glasses which show the rings, so as to cover a narrow central vertical zone passing through them all. The image of this zone upon the screen is crossed by portions of the iris-rings. Subjecting the reflected beam to prismatic analysis, the resultant spectrum may be regarded as an indefinite number of images of the zone placed side by side. In the image before dispersion we haveiris-rings, the extinction of the light being nowhere complete; but when the different colours are separated by dispersion, each colour is crossed transversely by its own system of dark interference bands, which become gradually closer with the increasing refrangibility of the light. The complete spectrum,therefore, appears furrowed by a system of continuous dark bands, crossing the colours transversely, and approaching each other as they pass from red to blue.

In the case of the plate of selenite, a slit is placed in front of the polarizer, and the film of selenite is held close to the slit, so that the light passes through the central zone of the film. As in the case of Newton's rings, the image of the zone is crossed by iris-coloured bands; but when subjected to prismatic dispersion, the light of the zone yields a spectrum furrowed by bands of complete darkness exactly as in the case of Newton's rings and for a similar reason. This is the beautiful effect described by Mr. Spottiswoode as the fanlike arrangement of the bands—the fan opening out at the red end of the spectrum.

The diffraction fringes described in Lecture II., instead of being formed on the retina, may be formed on a screen, or upon ground glass, when they can be looked at through a magnifying lens from behind, or they can be observed in the air when the ground glass is removed. Instead of permitting them to form on the retina, we will suppose them formed on a screen. This places us in a condition to understand, even without trigonometry, the solution of the important problem of measuringthe lengthof a wave of light.

Fig. 57.Fig. 57.

We will suppose the screen so distant that the rays falling upon it from the two margins of the slit are sensibly parallel. We have learned in Lecture II. that the first of the dark bands corresponds to a difference of marginal path of one undulation; the second dark band to a difference of path of two undulations; the third dark band to a difference of three undulations, and so on. Now the angular distance of the bands from the centre is capable of exact measurement; this distance depending, as already stated, on the width of the slit. With a slit 1.35 millimeter wide,[29]Schwerd found the angular distance of the first dark band from the centre of the field to be 1'38"; the angular distances of the second, third, fourth dark bands being twice, three times, four times this quantity.

Let A B, fig. 57, be the plate in which the slit is cut, and C D the grossly exaggerated width of the slit, with the beam of red light proceeding from it at the obliquity corresponding to the first dark band. Let fall a perpendicular from one edge, D, of the slit on the marginal ray of the other edge atd. The distance, Cd, between the foot of this perpendicular and the other edge is the length of a wave of the light. The angle C Dd, moreover, being equal to R C R', is, in the case now under consideration, 1'38". From the centre D, with the width D C as radius, describe a semicircle; its radius D C being 1.35 millimeter, the length of this semicircle is found by an easy calculation to be 4.248 millimeters. The length Cdis so small that it sensibly coincides with the arc of the circle. Hence the length of the semicircle is to the length Cdof the wave as 180° to1'38", or, reducing all to seconds, as 648,000" to 98". Thus, we have the proportion—

648,000 : 98 :: 4.248 to the wave-length Cd.

Making the calculation, we find the wave-length for this particular kind of light to be 0.000643 of a millimeter, or 0.000026 of an inch.

FOOTNOTES:[1]Among whom may be especially mentioned the late Sir Edmund Head, Bart., with whom I had many conversations on this subject.[2]At whose hands it gives me pleasure to state I have always experienced honourable and liberal treatment.[3]One of the earliest of these came from Mr. John Amory Lowell of Boston.[4]It will be subsequently shown how this simple apparatus may be employed to determine the 'polarizing angle' of a liquid.[5]From this principle Sir John Herschel deduces in a simple and elegant manner the fundamental law of reflection.—SeeFamiliar Lectures, p. 236.[6]The low dispersive power of water masks, as Helmholtz has remarked, the imperfect achromatism of the eye. With the naked eye I can see a distant blue disk sharply defined, but not a red one. I can also see the lines which mark the upper and lower boundaries of a horizontally refracted spectrum sharp at the blue end, but ill-defined at the red end. Projecting a luminous disk upon a screen, and covering one semicircle of the aperture with a red and the other with a blue or green glass, the difference between the apparent sizes of the two semicircles is in my case, and in numerous other cases, extraordinary. Many persons, however, see the apparent sizes of the two semicircles reversed. If with a spectacle glass I correct the dispersion of the red light over the retina, then the blue ceases to give a sharply defined image. Thus examined, the departure of the eye from achromatism appears very gross indeed.[7]Both in foliage and in flowers there are striking differences of absorption. The copper beech and the green beech, for example, take in different rays. But the very growth of the tree is due to some of the rays thus taken in. Are the chemical rays, then, the same in the copper and the green beech? In two such flowers as the primrose and the violet, where the absorptions, to judge by the colours, are almost complementary, are the chemically active rays the same? The general relation of colour to chemical action is worthy of the application of the method by which Dr. Draper proved so conclusively the chemical potency of the yellow rays of the sun.[8]Young, Helmholtz, and Maxwell reduce all differences of hue to combinations in different proportions of three primary colours. It is demonstrable by experiment that from the red, green, and violetallthe other colours of the spectrum may be obtained.Some years ago Sir Charles Wheatstone drew my attention to a work by Christian Ernst Wünsch, Leipzig 1792, in which the author announces the proposition that there are neither five nor seven, but only three simple colours in white light. Wünsch produced five spectra, with five prisms and five small apertures, and he mixed the colours first in pairs, and afterwards in other ways and proportions. His result is that red is asimplecolour incapable of being decomposed; that orange is compounded of intense red and weak green; that yellow is a mixture of intense red and intense green; that green is asimplecolour; that blue is compounded of saturated green and saturated violet; that indigo is a mixture of saturated violet and weak green; while violet is a puresimplecolour. He also finds that yellow and indigo blue producewhiteby their mixture. Yellow mixed with bright blue (Hochblau) also produces white, which seems, however, to have a tinge of green, while the pigments of these two colours when mixed always give a more or less beautiful green, Wünsch very emphatically distinguishes the mixture of pigments from that of lights. Speaking of the generation of yellow, he says, 'I say expresslyred and green light, because I am speaking about light-colours (Lichtfarben), and not about pigments.' However faulty his theories may be, Wünsch's experiments appear in the main to be precise and conclusive. Nearly ten years subsequently, Young adopted red, green, and violet as the three primary colours, each of them capable of producing three sensations, one of which, however, predominates over the two others. Helmholtz adopts, elucidates, and enriches this notion. (Popular Lectures, p. 249. The paper of Helmholtz on the mixture of colours, translated by myself, is published in thePhilosophical Magazinefor 1852. Maxwell's memoir on the Theory of Compound Colours is published in thePhilosophical Transactions, vol. 150, p. 67.)[9]The following charming extract, bearing upon this point, was discovered and written out for me by my deeply lamented friend Dr. Bence Jones, when Hon. Secretary to the Royal Institution:—'In every kind of magnitude there is a degree or sort to which our sense is proportioned, the perception and knowledge of which is of the greatest use to mankind. The same is the groundwork of philosophy; for, though all sorts and degrees are equally the object of philosophical speculation, yet it is from those which are proportioned to sense that a philosopher must set out in his inquiries, ascending or descending afterwards as his pursuits may require. He does well indeed to take his views from many points of sight, and supply the defects of sense by a well-regulated imagination; nor is he to be confined by any limit in space or time; but, as his knowledge of Nature is founded on the observation of sensible things, he must begin with these, and must often return to them to examine his progress by them. Here is his secure hold: and as he sets out from thence, so if he likewise trace not often his steps backwards with caution, he will be in hazard of losing his way in the labyrinths of Nature.'—(Maclaurin: An Account of Sir I. Newton's Philosophical Discoveries. Written 1728; second edition, 1750; pp. 18, 19.)[10]I do not wish to encumber the conception here with the details of the motion, but I may draw attention to the beautiful model of Prof. Lyman, wherein waves are shown to be produced by thecircularmotion of the particles. This, as proved by the brothers Weber, is the real motion in the case of water-waves.[11]Copied from Weber'sWellenlehre.[12]SeeLectures on Sound, 1st and 2nd ed., Lecture VII.; and 3rd ed., Chap. VIII. Longmans.[13]Boyle's Works, Birch's edition, p. 675.[14]Page 743.[15]The beautiful plumes produced by water-crystallization have been successfully photographed by Professor Lockett.[16]In a little volume entitled 'Forms of Water,' I have mentioned that cold iron floats upon molten iron. In company with my friend Sir William Armstrong, I had repeated opportunities of witnessing this fact in his works at Elswick, 1863. Faraday, I remember, spoke to me subsequently of the perfection of iron castings as probably due to the swelling of the metal on solidification. Beyond this, I have given the subject no special attention; and I know that many intelligent iron-founders doubt the fact of expansion. It is quite possible that the solid floats because it is notwettedby the molten iron, its volume being virtually augmented by capillary repulsion. Certain flies walk freely upon water in virtue of an action of this kind. With bismuth, however, it is easy to burst iron bottles by the force of solidification.[17]This beautiful law is usually thus expressed:The index of refraction of any substance is the tangent of its polarizing angle. With the aid of this law and an apparatus similar to that figured at page 15, we can readily determine the index of refraction of any liquid. The refracted and reflected beams being visible, they can readily be caused to inclose a right angle. The polarizing angle of the liquid may be thus found with the sharpest precision. It is then only necessary to seek out its natural tangent to obtain the index of refraction.[18]Whewell.[19]Removed from us since these words were written.[20]The only essay known to me on the Undulatory Theory, from the pen of an American writer, is an excellent one by President Barnard, published in the Smithsonian Report for 1862.[21]Boyle's Works, Birch's edition, vol. i. pp, 729 and 730.[22]Werke, B. xxix. p. 24.[23]Defined in Lecture I.[24]This circumstance ought not to be lost sight of in the examination of compound spectra. Other similar instances might be cited.[25]The dark band produced when the sodium is placed within the lamp was observed on the same occasion. Then was also observed for the first time the magnificent blue band of lithium which the Bunsen's flame fails to bring out.[26]New York: for more than a decade no such weather had been experienced. The snow was so deep that the ordinary means of locomotion were for a time suspended.[27]'Il faut reconnaître que parmi les peuples civilisés de nos jours il en est pen chez qui les hautes sciences aient fait moins de progrès qu'aux États-Unis, ou qui aient fourni moins de grands artistes, de poëtes illustres et de célèbres écrivains.' (De la Démocratie en Amérique, etc. tome ii. p. 36.)[28]At these points the two rectangular vibrations into which the original polarized ray is resolved by the plates of gypsum, act upon each other like the two rectangular impulses imparted to our pendulum in Lecture IV., one being given when the pendulum is at the limit of its swing. Vibration is thus converted into rotation.[29]The millimeter is about 1/25th of an inch.

FOOTNOTES:

[1]Among whom may be especially mentioned the late Sir Edmund Head, Bart., with whom I had many conversations on this subject.

[2]At whose hands it gives me pleasure to state I have always experienced honourable and liberal treatment.

[3]One of the earliest of these came from Mr. John Amory Lowell of Boston.

[4]It will be subsequently shown how this simple apparatus may be employed to determine the 'polarizing angle' of a liquid.

[5]From this principle Sir John Herschel deduces in a simple and elegant manner the fundamental law of reflection.—SeeFamiliar Lectures, p. 236.

[6]The low dispersive power of water masks, as Helmholtz has remarked, the imperfect achromatism of the eye. With the naked eye I can see a distant blue disk sharply defined, but not a red one. I can also see the lines which mark the upper and lower boundaries of a horizontally refracted spectrum sharp at the blue end, but ill-defined at the red end. Projecting a luminous disk upon a screen, and covering one semicircle of the aperture with a red and the other with a blue or green glass, the difference between the apparent sizes of the two semicircles is in my case, and in numerous other cases, extraordinary. Many persons, however, see the apparent sizes of the two semicircles reversed. If with a spectacle glass I correct the dispersion of the red light over the retina, then the blue ceases to give a sharply defined image. Thus examined, the departure of the eye from achromatism appears very gross indeed.

[7]Both in foliage and in flowers there are striking differences of absorption. The copper beech and the green beech, for example, take in different rays. But the very growth of the tree is due to some of the rays thus taken in. Are the chemical rays, then, the same in the copper and the green beech? In two such flowers as the primrose and the violet, where the absorptions, to judge by the colours, are almost complementary, are the chemically active rays the same? The general relation of colour to chemical action is worthy of the application of the method by which Dr. Draper proved so conclusively the chemical potency of the yellow rays of the sun.

[8]Young, Helmholtz, and Maxwell reduce all differences of hue to combinations in different proportions of three primary colours. It is demonstrable by experiment that from the red, green, and violetallthe other colours of the spectrum may be obtained.Some years ago Sir Charles Wheatstone drew my attention to a work by Christian Ernst Wünsch, Leipzig 1792, in which the author announces the proposition that there are neither five nor seven, but only three simple colours in white light. Wünsch produced five spectra, with five prisms and five small apertures, and he mixed the colours first in pairs, and afterwards in other ways and proportions. His result is that red is asimplecolour incapable of being decomposed; that orange is compounded of intense red and weak green; that yellow is a mixture of intense red and intense green; that green is asimplecolour; that blue is compounded of saturated green and saturated violet; that indigo is a mixture of saturated violet and weak green; while violet is a puresimplecolour. He also finds that yellow and indigo blue producewhiteby their mixture. Yellow mixed with bright blue (Hochblau) also produces white, which seems, however, to have a tinge of green, while the pigments of these two colours when mixed always give a more or less beautiful green, Wünsch very emphatically distinguishes the mixture of pigments from that of lights. Speaking of the generation of yellow, he says, 'I say expresslyred and green light, because I am speaking about light-colours (Lichtfarben), and not about pigments.' However faulty his theories may be, Wünsch's experiments appear in the main to be precise and conclusive. Nearly ten years subsequently, Young adopted red, green, and violet as the three primary colours, each of them capable of producing three sensations, one of which, however, predominates over the two others. Helmholtz adopts, elucidates, and enriches this notion. (Popular Lectures, p. 249. The paper of Helmholtz on the mixture of colours, translated by myself, is published in thePhilosophical Magazinefor 1852. Maxwell's memoir on the Theory of Compound Colours is published in thePhilosophical Transactions, vol. 150, p. 67.)

Some years ago Sir Charles Wheatstone drew my attention to a work by Christian Ernst Wünsch, Leipzig 1792, in which the author announces the proposition that there are neither five nor seven, but only three simple colours in white light. Wünsch produced five spectra, with five prisms and five small apertures, and he mixed the colours first in pairs, and afterwards in other ways and proportions. His result is that red is asimplecolour incapable of being decomposed; that orange is compounded of intense red and weak green; that yellow is a mixture of intense red and intense green; that green is asimplecolour; that blue is compounded of saturated green and saturated violet; that indigo is a mixture of saturated violet and weak green; while violet is a puresimplecolour. He also finds that yellow and indigo blue producewhiteby their mixture. Yellow mixed with bright blue (Hochblau) also produces white, which seems, however, to have a tinge of green, while the pigments of these two colours when mixed always give a more or less beautiful green, Wünsch very emphatically distinguishes the mixture of pigments from that of lights. Speaking of the generation of yellow, he says, 'I say expresslyred and green light, because I am speaking about light-colours (Lichtfarben), and not about pigments.' However faulty his theories may be, Wünsch's experiments appear in the main to be precise and conclusive. Nearly ten years subsequently, Young adopted red, green, and violet as the three primary colours, each of them capable of producing three sensations, one of which, however, predominates over the two others. Helmholtz adopts, elucidates, and enriches this notion. (Popular Lectures, p. 249. The paper of Helmholtz on the mixture of colours, translated by myself, is published in thePhilosophical Magazinefor 1852. Maxwell's memoir on the Theory of Compound Colours is published in thePhilosophical Transactions, vol. 150, p. 67.)

[9]The following charming extract, bearing upon this point, was discovered and written out for me by my deeply lamented friend Dr. Bence Jones, when Hon. Secretary to the Royal Institution:—'In every kind of magnitude there is a degree or sort to which our sense is proportioned, the perception and knowledge of which is of the greatest use to mankind. The same is the groundwork of philosophy; for, though all sorts and degrees are equally the object of philosophical speculation, yet it is from those which are proportioned to sense that a philosopher must set out in his inquiries, ascending or descending afterwards as his pursuits may require. He does well indeed to take his views from many points of sight, and supply the defects of sense by a well-regulated imagination; nor is he to be confined by any limit in space or time; but, as his knowledge of Nature is founded on the observation of sensible things, he must begin with these, and must often return to them to examine his progress by them. Here is his secure hold: and as he sets out from thence, so if he likewise trace not often his steps backwards with caution, he will be in hazard of losing his way in the labyrinths of Nature.'—(Maclaurin: An Account of Sir I. Newton's Philosophical Discoveries. Written 1728; second edition, 1750; pp. 18, 19.)

[9]The following charming extract, bearing upon this point, was discovered and written out for me by my deeply lamented friend Dr. Bence Jones, when Hon. Secretary to the Royal Institution:—

'In every kind of magnitude there is a degree or sort to which our sense is proportioned, the perception and knowledge of which is of the greatest use to mankind. The same is the groundwork of philosophy; for, though all sorts and degrees are equally the object of philosophical speculation, yet it is from those which are proportioned to sense that a philosopher must set out in his inquiries, ascending or descending afterwards as his pursuits may require. He does well indeed to take his views from many points of sight, and supply the defects of sense by a well-regulated imagination; nor is he to be confined by any limit in space or time; but, as his knowledge of Nature is founded on the observation of sensible things, he must begin with these, and must often return to them to examine his progress by them. Here is his secure hold: and as he sets out from thence, so if he likewise trace not often his steps backwards with caution, he will be in hazard of losing his way in the labyrinths of Nature.'—(Maclaurin: An Account of Sir I. Newton's Philosophical Discoveries. Written 1728; second edition, 1750; pp. 18, 19.)

[10]I do not wish to encumber the conception here with the details of the motion, but I may draw attention to the beautiful model of Prof. Lyman, wherein waves are shown to be produced by thecircularmotion of the particles. This, as proved by the brothers Weber, is the real motion in the case of water-waves.

[11]Copied from Weber'sWellenlehre.

[12]SeeLectures on Sound, 1st and 2nd ed., Lecture VII.; and 3rd ed., Chap. VIII. Longmans.

[13]Boyle's Works, Birch's edition, p. 675.

[14]Page 743.

[15]The beautiful plumes produced by water-crystallization have been successfully photographed by Professor Lockett.

[16]In a little volume entitled 'Forms of Water,' I have mentioned that cold iron floats upon molten iron. In company with my friend Sir William Armstrong, I had repeated opportunities of witnessing this fact in his works at Elswick, 1863. Faraday, I remember, spoke to me subsequently of the perfection of iron castings as probably due to the swelling of the metal on solidification. Beyond this, I have given the subject no special attention; and I know that many intelligent iron-founders doubt the fact of expansion. It is quite possible that the solid floats because it is notwettedby the molten iron, its volume being virtually augmented by capillary repulsion. Certain flies walk freely upon water in virtue of an action of this kind. With bismuth, however, it is easy to burst iron bottles by the force of solidification.

[17]This beautiful law is usually thus expressed:The index of refraction of any substance is the tangent of its polarizing angle. With the aid of this law and an apparatus similar to that figured at page 15, we can readily determine the index of refraction of any liquid. The refracted and reflected beams being visible, they can readily be caused to inclose a right angle. The polarizing angle of the liquid may be thus found with the sharpest precision. It is then only necessary to seek out its natural tangent to obtain the index of refraction.

[18]Whewell.

[19]Removed from us since these words were written.

[20]The only essay known to me on the Undulatory Theory, from the pen of an American writer, is an excellent one by President Barnard, published in the Smithsonian Report for 1862.

[21]Boyle's Works, Birch's edition, vol. i. pp, 729 and 730.

[22]Werke, B. xxix. p. 24.

[23]Defined in Lecture I.

[24]This circumstance ought not to be lost sight of in the examination of compound spectra. Other similar instances might be cited.

[25]The dark band produced when the sodium is placed within the lamp was observed on the same occasion. Then was also observed for the first time the magnificent blue band of lithium which the Bunsen's flame fails to bring out.

[26]New York: for more than a decade no such weather had been experienced. The snow was so deep that the ordinary means of locomotion were for a time suspended.

[27]'Il faut reconnaître que parmi les peuples civilisés de nos jours il en est pen chez qui les hautes sciences aient fait moins de progrès qu'aux États-Unis, ou qui aient fourni moins de grands artistes, de poëtes illustres et de célèbres écrivains.' (De la Démocratie en Amérique, etc. tome ii. p. 36.)

[28]At these points the two rectangular vibrations into which the original polarized ray is resolved by the plates of gypsum, act upon each other like the two rectangular impulses imparted to our pendulum in Lecture IV., one being given when the pendulum is at the limit of its swing. Vibration is thus converted into rotation.

[29]The millimeter is about 1/25th of an inch.

Absorption, principles of,199Airy, Sir George, severity and conclusiveness of his proofs,209Alhazen, his inquiry respecting light,14,207Analyzer, polarizer and,127——recompounding of the two systems of waves by the analyzer,129Ångström, his paper on spectrum analysis,202Arago, François, and Dr. Young,50——his discoveries respecting light,208Atomic polarity,93-96Bacon, Roger, his inquiry respecting light,14,207Bartholinus, Erasmus, on Iceland spar,112Bérard on polarization of heat,180Blackness, meaning of,32Boyle, Robert, his observations on colours,65,66——his remarks on fluorescence,163,164Bradley, James, discovers the aberration of light,21,22Brewster, Sir David, his chief objection to the undulatory theory of light,47Brewster, Sir David, his discovery in biaxal crystals,209Brougham, Mr. (afterwards Lord), ridicules Dr. T. Young's speculations,50,51Cæsium, discovery of,193Calorescence,174Clouds, actinic,152-154——polarization of,155Colours of thin plates,64——Boyle's observations on,65,66——Hooke on the colours of thin plates,67——of striated surfaces,89,90Comet of 1680, Newton's estimate of the temperature of,168Crookes, Mr., his discovery of thallium,193Crystals, action of, upon light,98——built by polar force,98——illustrations of crystallization,99——architecture of, considered as an introduction to their action upon light,98——bearings of crystallization upon optical phenomena,106Crystals, rings surrounding the axes of, uniaxal and biaxal,145Cuvier on ardour for knowledge,220De Tocqueville, writings of,215,222,223Descartes, his explanation of the rainbow,24,25——his ideas respecting the transmission of light,43——his notion of light,207Diamond, ignition of a, in oxygen,169Diathermancy,173Diffraction of light, phenomena of,78——bands,78,79——explanation of,80——colours produced by,89Dollond, his experiments on achromatism,28Draper, Dr., his investigation on heat,172Drummond light, spectrum of,195Earth, daily orbit of,74Electric beam, heat of the,168Electricity, discoveries in,217,218Emission theory of light, bases of the,45——Newton espouses the theory, and the results of this espousal,77Ether, Huyghens and Euler advocate and defend the conception of an,48,58——objected to by Newton,58Euler espouses and defends the conception of an ether,48,58Eusebius on the natural philosophers of his time,13Expansion by cold,104Experiment, uses of,3Eye, the, its imperfections, grown for ages towards perfection,8——imperfect achromatism of the,29,noteFaraday, Michael, his discovery of magneto-electricity,218'Fits,' theory of,73——its explanation of Newton's rings,74——overthrow of the theory,77Fizeau determines the velocity of light,22Fluorescence, Stokes's discovery of,161——the name,174Forbes, Professor, polarizes and depolarizes heat,180Foucault, determines the velocity of light,22——his experiments on absorption,197,198Fraunhofer, his theoretical calculations respecting diffraction,87——his lines,193———their explanation by Kirchhoff,193Fresnel, and Dr. Young,50——his theoretical calculations respecting diffraction,87——his mathematical abilities and immortal name,210Goethe on fluorescence,165Gravitation, origin of the notion of the attraction of,92——strength of the theory of,148Grimaldi, his discovery with respect to light,56——Young's generalizations of,56Hamilton, Sir William, of Dublin, his discovery of conical refraction,209Heat, generation of,6——Dr. Draper's investigation respecting,171Helmholtz, his estimate of the genius of Young,50——on the imperfect achromatism of the eye,29,note,31——reveals the cause of green in the case of pigments,37Henry, Professor Joseph, his invitation,2Herschel, Sir John, his theoretical calculations respecting diffraction,87——first notices and describes the fluorescence of sulphate of quinine,165——his experiments on spectra,201Herschel, Sir William, his experiments on the heat of the various colours of the solar spectrum,171Hooke, Robert, on the colours of thin plates,67——his remarks on the idea that light and heat are modes of motion,68Horse-chestnut bark, fluorescence of,165Huggins, Dr., his labours,205Huyghens advocates the conception of ether,48,58——his celebrated principle,83Huyghens on the double refraction of Iceland spar,112Iceland spar,109——double refraction caused by,110——this double refraction first treated by Erasmus Bartholinus,112——character of the beams emergent from,114——tested by tourmaline,116——Knoblauch's demonstration of the double refraction of,185Ice-lens, combustion through,167Imagination, scope of the,42——note by Maclaurin on this point, 43noteJanssen, M., on the rose-coloured solar prominences,204Jupiter, Roemer's observations of the moons of,20Jupiter's distance from the sun,20Kepler, his investigations on the refraction of light,14,207Kirchhoff, Professor, his explanation of Fraunhofer's lines,193——his precursors,201——his claims,203Knoblauch, his demonstration of the double refraction of heat of Iceland spar,185Lactantius, on the natural philosophers of his time,13Lamy, M., isolates thallium in ingots,193Lesley, Professor, his invitation,2Light familiar to the ancients,5——generation of,6,7——spherical aberration of,8——the rectilineal propagation of, and mode of producing it,9——illustration showing that the angle of incidence is equal to the angle of reflection,10,11——sterility of the Middle Ages,13——history of refraction,14——demonstration of the fact of refraction,14——partial and total reflection of,16-20——velocity of,20——Bradley's discovery of the aberration of light,21,22——principle of least time,23——Descartes and the rainbow,24——Newton's analysis of,26,27——synthesis of white light,30——complementary colours,31——yellow and blue lights produce white by their mixture,31——what is the meaning of blackness?32——analysis of the action of pigments upon,33——absorption,34——mixture of pigments contrasted with mixture of lights,37——Wünsch on three simple colours in white light,39note——Newton arrives at the emission theory,45——Young's discovery of the undulatory theory,49——illustrations of wave-motion,58——interference of sound-waves,58——velocity of,60——principle of interference of waves of,61——phenomena which first suggested the undulatory theory62-69——soap-bubbles and their colours,62-65——Newton's rings,69-77——his espousal of the emission theory, and the results of this espousal,77——transmitted light,77——diffraction,77,89——origin of the notion of the attraction of gravitation,92——polarity, how generated,93——action of crystals upon,98——refraction of,106——elasticity and density,108——double refraction,109——chromatic phenomena produced by crystals in polarized,121——the Nicol prism,122——mechanism of,125——vibrations,125——composition and resolution of vibrations,128——polarizer and analyzer,127——recompounding the two systems of waves by the analyzer,129——interference thus rendered possible,131——chromatic phenomena produced by quartz,139——magnetization, of,141——rings surrounding the axes of crystals,143——colour and polarization of sky,149,154——range of vision incommensurate with range of radiation,159——effect of thallene on the spectrum, 162——fluorescence,162——transparency,167——the ultra-red rays,170——part played in Nature by these rays,175——conversion of heat-rays into light-rays,176——identity of radiant heat and,177——polarization of heat,180——principles of spectrum analysis,189——spectra of incandescent vapours,190——Fraunhofer's lines, and Kirchhoff's explanation of them,193——solar chemistry,195-197——demonstration of analogy between sound and,198,199——Kirchhoff and his precursors,201——rose-coloured solar prominences,204——results obtained by various workers,205——summary and conclusion,206——polarized, the spectra of,227——measurement of the waves of,234Lignum Nephriticum, fluorescence of,164Lloyd, Dr., on polarization of heat,180,209Lockyer, Mr., on the rose-coloured solar prominences,205Lycopodium, diffraction effects caused by the spores of,88Magnetization of light,141Malus, his discovery respecting reflected light through Iceland spar,115——discovers the polarization of light by reflection,208Masson, his essay on the bands of the induction spark,202Melloni, on the polarization of heat,180Metals, combustion of,5,6——spectrum analysis of,190——spectrum bands proved by Bunsen and Kirchhoff to be characteristic of the vapour of,192Mill, John Stuart, his scepticism regarding the undulatory theory,149Miller, Dr., his drawings and descriptions of the spectra of various coloured flames,201Morton, Professor, his discovery of thallene,162Mother-of-pearl, colours of,90Nature, a savage's interpretation of,4Newton, Sir Isaac, his experiments on the composition of solar light,26——his spectrum,27——dispersion,27——arrives at the emission theory of light,45——his objection to the conception of an ether espoused and defended by Huyghens and Euler,58——his optical career,70——his rings,69-77——his rings explained by the theory of 'fits,'73——espouses the emission theory,77——effects of this espousal,77——his idea of gravitation,92——his errors,208Nicol prism, the,122Ocean, colour of the,35Œrsted, discovers the deflection of a magnetic needle by an electric current,176Optics, science of,4Pasteur referred to,219Physical theories, origin of,41-44Pigments, analysis of the action of, upon light,33——mixture of, contrasted with mixture of lights,37——Helmholtz reveals the cause of the green in the case of mixed blue and yellow pigments,37——impurity of natural colours,37Pitch of sound,59Plücker, his drawings of spectra,202Polariscope, stained glass in the, 130,131——unannealed glass in the,136Polarity, notion of, how generated,93——atomic,93-96——structural arrangements due to,96——polarization of light,112——tested by tourmaline,116——and by reflection and refraction,119——depolarization,120Polarization of light,112——circular,140——sky-light,149,157——of artificial sky,156——of radiant heat,180Polarizer and analyzer,127Poles of a magnet,93Powell, Professor, on polarization of heat,180Prism, the Nicol,122Quartz, chromatic phenomena produced by,139Radiant heat,172——diathermancy, or perviousness to radiant heat,173——conversion of heat-rays into light rays,174——formation of invisible heat-images,179——polarization of,180——double refraction,182——magnetization of,184Rainbow, Descartes' explanation of the,24Refraction, demonstration of,14Refraction of light,106——double,109Reflection, partial and total,16-20Respighi, results obtained by,205Ritter, his discovery of the ultraviolet rays of the sun,159Roemer, Olav, his observations of Jupiter's moons,20——his determination of the velocity of light,21Rubidium, discovery of,193Rusting of iron, what it is,5Schwerd, his observations respecting diffraction,87Science, growth of,176,203Scoresby, Dr., succeeds in exploding gunpowder by the sun's rays conveyed by large lenses of ice,167Secchi, results obtained by,205Seebeck, Thomas, discovers thermo-electricity,176——discovers the polarization of light by tourmaline,208Selenite, experiments with thick and thin plates of,124Silver spectrum, analysis of,190,191Sky-light, colour and polarization of,149,154——generation of artificial skies,152Snell, Willebrord, his discovery,14——his law,15,24Soap-bubbles and their colours,63,65Sound, early notions of the ancients respecting,51——interference of waves of,58——pitch of,59——analogies of light and,56——demonstration of analogy between, and light,198,199Sonorous vibrations, action of,134Spectrum analysis, principles of,189Spectra of incandescent vapours,190——discontinuous,191,192——of polarized light,227Spectrum bands proved by Bunsen and Kirchhoff to be characteristic of the vapour,192——its capacity as an agent of discovery,193——analysis of the sun and stars,193Spottiswoode, Mr. William,123,227Stewart, Professor Balfour,202Stokes, Professor, results of his examination of substances excited by the ultra-violet waves,161——his discovery of fluorescence,162——on fluorescence,165——nearly anticipates Kirchhoff's discovery,198,202Striated surfaces, colours of,89Sulphate of quinine first noticed and described by Sir John Herschel,165Sun, chemistry of the,195Sun, rose-coloured solar prominences,204Talbot, Mr., his experiments,201Tartaric acid, irregular crystallization of, and its effects,131Thallene, its effect on the spectrum,162Thallium, spectrum analysis of,190,191——discovery of,193——isolated in ingots by M. Lamy,193Theory, relation of, to experience,91Thermo-electric pile,176Thermo-electricity, discovery of,176Tombeline, Mont, inverted image of,19Tourmaline, polarization of light by means of,112Transmitted light, reason for,77Transparency, remarks on,167Ultra-violet sun-rays, discovered by Ritter,159——effects of,160Ultra-red rays of the solar spectrum,171——part played by the,173Undulatory theory of light, bases of the,47——Sir David Brewster's chief objection to the,47Undulatory theory of light, Young's foundation of the,49——phenomena which first suggested the,62,69——Mr. Mill's scepticism regarding the,143——a demonstrated verity in the hands of Young,210Vassenius describes the rose-coloured solar prominences in 1733,204Vitellio, his skill and conscientiousness,14——his investigations respecting light,207Voltaic battery, use of, and its production of heat,6,7Water, deportment of, considered and explained,105,106Waves of water,51——length of a wave,52——interference of waves,53-55Wertheim, M., his instrument for the determination of strains and pressures by the colours of polarized light,134Wheatstone, Sir Charles, his analysis of the light of the electric spark,202Whirlpool Rapids, illustration of the principle of the interference of waves at the,55Willigen, Van der, his drawings of spectra,202Wollaston, Dr., first observes lines in solar spectrum,193——discovers the rings of Iceland spar,209Woodbury, Mr., on the impurity of natural colours,37Wünsch, Christian Ernst, on the three simple colours in white lights,39note——his experiments,39noteYoung, Dr. Thomas, his discovery of Egyptian hieroglyphics,49——and the undulatory theory of light,49——Helmholtz's estimate of him,50——ridiculed by Brougham in the 'Edinburgh Review,'50——generalizes Grimaldi's observation on light,56,57——photographs the ultra-violet rings of Newton,160

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