figure 57.
figure 57.
The stands for these telescopes, and the manner in which they are fitted up for observation are represented in figures 57, 58, and 59. Fig. 57 representseither the 2½ or the 3½ feet telescopes mounted on a plain brass stand, to be placed on a table. A is the long eye-piece for land objects, and B the small eye-piece for astronomical observation, which is composed of two lenses, and represents the object in an inverted position. These eye-pieces are screwed on, as occasion requires, at E, the eye-end of the telescope. The shorter of the two astronomical eye-tubes which accompany this telescope, produces the highest magnifying power. For adjusting the telescope to distinct vision, there is a brass knob or button ata, which moves a piece of rack-work connected with the eye-tube, which must be turned either one way or the other till the object appears distinctly; and different eyes frequently require a different adjustment.
Fig. 58, represents a 5 feet telescope fitted up for astronomical observations. It is mounted on a mahogany stand, the three legs of which are made to close up together by means of the brass frameaaa, which is composed of three bars, connected with three joints in the centre, and three other joints, connected with the three mahogany bars. It is furnished with an apparatus for equatorial motions. The brass pin is made to move round in the brass socketb, and may be tightened by means of the finger screwd, when the telescope is directed nearly to the object intended to be viewed. This socket may be set perpendicular to the horizon, or to any other required angle; and the quantity of the angle is ascertained by the divided arc, and the instrument made fast in that position by the screwe. If this socket be set to the latitude of the place of observation, and the plane of this arc be turned so as to be in the plane of the meridian, the socketbbeing fixed to the inclinationof the pole of the earth, the telescope when turned in this socket, will have an equatorial motion, so that celestial objects may be always kept in view, when this equatorial motion is performed. The two handles atkare connected with rack-work, intended to move the telescope in any required direction. The two sets of brass sliding rodsiiare intended to render the telescope as steady as possible, and to elevate and depress it at pleasure, and are so constructed as to slide into each other with the utmost ease.
figure 58.
figure 58.
TheFinderis placed at AE, either on the top or the left side of the tube of the telescope.When high magnifying powers are applied to any telescope, it is sometimes difficult, on account of the smallness of the field of view, to direct the main tube of the telescope to the object. But the Finder, which is a telescope with a small power, and consequently has a large field of view—when directed to any object, it is easily found, and being brought to the centre of the field, where two cross hairs intersect each other, it will then be seen in the larger telescope. B is the eye-tube for terrestrial objects, containing 4 glasses, and C, one of the astronomical eye-pieces. A socket is represented atg, containing a stained glass, which is screwed to any of the eye-pieces, to protect the eye from the glare of light, when viewing the spots of the sun. The brass nut abovef, is intended for the adjustment of the eye-piece to distinct vision. The 3½ feet telescope is sometimes mounted in this form.
Fig. 59, represents a 5 or 6 feet telescope, mounted on a stand of a new construction by Dollond. It possesses the advantage of supporting the telescope in two places, which renders it extremely steady—a property of great importance when viewing celestial objects with high magnifying powers. It possesses likewise, the advantage of enabling the observer to continue seated at the same height from the floor, although the telescope be raised to any altitude—the elevation being entirely at the object end, although it may be changed from the horizon to the zenith. The frame-work is composed of bars of mahogany, and rests on three castors, two of which are made fast to their respective legs in the usual way, and the third stands under the middle of the lower horizontal bar that connects the two opposite legs, so that the frame has all the advantages of a tripod. Asit becomes very inconvenient to stoop to the eye end of a telescope, when the altitude of an object is considerable, and the centre of motion at the middle of the tube, this construction of a stand serves to remedy such inconvenience.
figure 59.
figure 59.
As some ingenious mechanics may feel a desire to attempt the construction of a compound achromatic object-glass, I shall here state some of the proportions of curvature of the concave and convex lenses, which serve to guide opticians in their construction of achromatic instruments. These proportions are various; and even when demonstrated to be mathematically correct, it is sometimes difficult to reduce them to practice, on account of the different powers of refraction and dispersion possessed by different discs of crown and flint-glass, and of the difficulty of producing by mechanical means, the exact curves which theory requires. The following table shows the radii of curvature of the different surfaces of the lenses necessary to forma double achromatic object-glass—it being supposed that the sine of refraction in the crown-glass is as 1.528 to 1, and in the flint as 1.5735 to 1; the ratio of their dispersive powers being as 1 to 1.524. It is also assumed that the curvatures of the concave lens are as 1 to 2, that is, that the one side of this lens is ground on a tool, the radius of which is double that of the other. The 1st column expresses the compound focus of the object-glass ininches; the 2nd column states the radius of theanteriorsurface of thecrown, and column 3rd, itsposteriorside. Column 4th expresses the radius of the anterior surface of theconcavelens, and column 5th its posterior surface, which, it will be observed, is exactly double that of the other.
From the above table it will be seen, that to construct, for example, a 30 inch compound object-glass, the radius of the anterior side of the crown must be 7½ inches, and that of the posterior side 11.63 inches; the radius of the anterior surface of the concave 10.428, and that of the posterior 20.856 inches. It may be proper to observe, that in these computations, the radius of the anterior surface of the concave is less than the posterior side of the convex, and consequently admits of its approach, without touching in the centre—a circumstance which always requires to be guarded against in the combination of achromatic glasses. The following table shows the radii of curvature of the lenses of atripleobject-glass, calculated from formula deduced by Dr. Robison of Edinburgh.
The following table contains the proportions of curvature, said to be employed by the London opticians.
From this table it appears, that the two convex lenses, have the same radii of their respective sides and that the concave flint lens has its two surfaces equally concave, so that a triple object-glass formed according to these proportions, would require only three pair of grinding tools. The following are the curves of the lenses of one of the best of Dollond’s achromatic telescopes, the focal length of the compound object-glass being 46 inches. Reckoning from the surface next the object—the radii of the crown-glass were 28 and 40 inches: the concave lens 20.9 inches, and the inner crown-glass lens, 28.4 and 28.4 inches. This telescope carried magnifying powers of from 100 to 200 times.
Although I have inserted the above tables, which might in some measure guide an ingenious artist, yet on the whole, a private amateur has little chance in succeeding in such attempts. The diversity of glasses, and the uncertainty of an unpractised workman’s producing the precise curvatures he intends, is so great, that the object-glass,for the most part, turns out different from his expectations. The great difficulty in the construction is to find the exact proportion of the dispersive powers of the crown and flint glass. The crown is pretty constant, but there are hardly two pots of flint glass which have the same dispersive power. Even if constant, it is difficult to measure it accurately; and an error in this greatly affects the instrument; because the focal distances of the lenses must be nearly as their dispersive powers. In the two preceding tables, the sine of incidence, in the crown glass, is supposed to be to the sine of refraction as 1.526 to 1; and in the flint glass, as 1.604 to 1. Opticians who make great numbers of lenses both of flint and crown glass, acquire, in time, a pretty good guess of the nature of the errors which may remain after they have finished an object-glass; and having many lenses intended to be of the same form, but unavoidably differing a little from it, they try several of the concaves with the two convexes, and finding one better than the rest, they make use of it to complete the set. In this way some of the best achromatic telescopes are frequently formed. I have sometimes found, when supplying a concave flint glass to a telescope where it happened to be wanting, that, of four or five concave lenses which appeared to be the same as to curvature and other properties, only one was found to produce a distinct and colourless image. Should any one, however, wish to attempt the construction of an achromatic lens, the best way for preventing disappointments in the result is, to procure avarietyof tables of the respective curvatures founded ondifferent conditions, and which, of course, require the surfaces of the several lenses to be of different curves.Having lenses of different radii at his command, and having glass of different refractive or dispersive powers, when one combination does not exactly suit, he may try another, and ultimately may succeed in constructing a good achromatic telescope; for, in many cases, it has been found that chance, or a happy combination of lenses by trial, has led to the formation of an excellent object-glass.
The best achromatic telescopes, when minutely examined, are found to be in some respects defective, on account of that slight degree of colour which, by the aberration of the rays, they give to objects, unless the object-glass be of small diameter. When we examine with attention a good achromatic telescope we find that it does not show white or luminous objects perfectly free from colour, their edges being tinged on one side with a claret-coloured fringe, and on the other with a green fringe. This telescope, therefore, required farther improvement, to get rid of these secondary colours, and Father Boscovich, to whom every branch of optics is much indebted, displayed much ingenuity in his attempts to attain this object. But it is to Dr. Blair, professor of astronomy in Edinburgh, that we are chiefly indebted for the first successful experiments by which this end was accomplished. By a judicious set of experiments, he proved that the quality of dispersing the rays in a greater degree than crown-glass, is not confined to a few mediums; but is possessed by a great variety of fluids, and by some of these in a most extraordinary degree. Having observed that when the extreme red and violet rays were perfectly united, the green were left out, he conceivedthe idea of making an achromatic concave lens which should refract the green less than the united red and violet, and an achromatic convex lens which should do the same, and as the concave lens refracted the outstanding greentothe axis, while the concave one refracted themfromthe axis, it followed, that, by a combination of these two opposite effects, the green would be united with the red and violet.
By means of an ingenious prismatic apparatus, he examined the optical properties of a great variety of fluids. The solutions of metals and semi-metals proved in all cases more dispersive than crown glass. Some of the salts, such as sal-ammoniac, greatly increased the dispersive power of water. The marine acid disperses very considerably, and this quality increases with its strength. The most dispersive fluids were accordingly found to be those in which this acid and the metals were combined. The chemical preparation calledcausticum antimoniale, or butter of antimony, in its most concentrated state, when it has just attracted sufficient humidity to render it fluid, possesses the quality of dispersing the rays in an astonishing degree. The great quantity of the semi-metal retained in solution, and the highly concentrated state of the marine acid, are considered as the cause of this striking effect. Corrosive sublimate of mercury, added to a solution ofsal-ammoniacumin water, possesses the next place to the butter of antimony among the dispersive fluids, which Dr. Blair examined. The essential oils were found to hold the next rank to metallic solutions, among fluids which possess the dispersive quality, particularly those obtained from bituminous minerals, as native petrolea, pit coal, and amber. The dispersive power of the essential oilof sassafras, and the essential oil of lemons, when genuine, were found to be not much inferior to any of these. But of all the fluids fitted for optical purposes, Dr. Blair found thatthe muriatic acid mixed with a metallic solution, or, in other words, a fluid in which the marine acid and metalline particles, hold a due proportion, most accurately suited his purpose. In a spectrum formed by this fluid the green were among the most refrangible rays, and when its dispersion was corrected by that of glass, there was produced an inverted secondary spectrum, that is, one in which the green was above, when it would have been below with a common medium. He therefore placed a concave lens of muriatic acid with a metallic solution between the two lenses, as in fig. 60, where AB is the concave fluid lens, CF a plano-convex lens, with its plane side next the object, and ED, a meniscus. With this object-glass the rays of different colours were bent from their rectilineal course with the same equality and regularity as in reflection.
figure 60.
figure 60.
Telescopes constructed with such object-glasses were examined by the late Dr. Robison and professor Playfair. The focal distance of the object-glass of one of these did not exceed 17 inches, and yet it bore an aperture of 3½ inches. They viewed some single and double stars and some common objects with this telescope; and found, that, in magnifying power, brightness, and distinctness,it was manifestly superior to one of Mr. Dollond of 42 inches focal length. They had most distinct vision of a star,when using an erecting eye-piece, which made this telescope magnify more than a 100 times; and they found the field of vision as uniformly distinct as with Dollond’s 42 inch telescope magnifying 46 times; and were led to admire the nice figuring and centering of the very deep eye-glasses which were necessary for this amplification. They saw double stars with a degree of perfection which astonished them. These telescopes, however, have never yet come into general use; and one reason perhaps, is, that they are much more apt to be deranged, than telescopes constructed of object-glasses which are solid. If any species of glass, or other solid transparent substance could be found with the same optical properties, instruments might perhaps be constructed of a larger size, and considerably superior to our best achromatic telescopes.23It is said that Mr. Blair, the son of Dr. Blair, some years ago, was engaged in prosecuting his father’s views, but I have not heard any thing respecting the result of his investigations.
Professor Barlow, not many years ago, suggested a new fluid telescope, which is deserving of attention; and, about the year 1829 constructed one of pretty large dimensions. The fluid he employs for this purpose is thesulphuret ofCarbon, which he found to be a substance which possessed every requisite he could desire. Its index is nearly the same as that of the best flint glass, with a dispersive power more than double. It is perfectly colourless, beautifully transparent, and although very expansible, possesses the same, or very nearly the same optical properties under all circumstances to which it is likely to be exposed in astronomical observations—except perhaps, direct observations on the solar disc, which will probably be found inadmissible. Mr. Barlow first constructed an object-glass with this fluid of 3 inches aperture, with which he could see the small star in Polaris with a power of 46, and with the higher powers several stars which are considered to require a good telescope, for example 70, Ï Ophinchi, 39 Bootis, the quadruple star ε Lyræ, ζ Aquarii, α Herculis, &c. He next constructed a 6 inch object-glass. With this instrument the small star in Polaris is so distinct and brilliant, with a power of 143, that its transit might be taken with the utmost certainty. As the mode of constructing these telescopes is somewhat novel, it may be expedient to enter somewhat into detail.
In the usual construction of achromatic telescopes, the two or three lenses composing the object-glass are brought into immediate contact; and in the fluid telescope of Dr. Blair, the construction was the same, the fluid having been enclosed in the object-glass itself. But in Mr. Barlow’s telescope, the fluid correcting lens is placed at a distance from the plate lens equal to half its focal length; and it might be carried still farther back, and yet possess dispersive power to render the object-glass achromatic. By this means the fluid lens—which is the most difficultpart of the construction—is reduced to one half or to less than one half of the size of the plate lens; consequently, to construct a telescope of 10 or 12 inches aperture involves no greater difficulty in the manipulation, than in making a telescope of the usual description of 5 or 6 inches aperture, except in the simple plate lens itself; and, hence, a telescope of this kind, of 10 or 12 feet length, will be equivalent in its focal power to one of 16 or 20 feet. By this means, the tube may be shortened several feet and yet possess a focal power more considerable than could be conveniently given to it on the usual principle of construction. This will be better understood from the annexed diagram. (fig. 61.)
figure 61.
figure 61.
In this figure ABCD represent the tube of the 6 inch telescope, CD, the plate object-glass, F the first focus of rays,dethe fluid concave lens, distant from the former 24 inches. The focal length MF being 48, and consequently, as 48 : 6 :: 24 : 3 inches, the diameter of the fluid lens. The resulting compound focus is 62.5 inches. It is obvious, therefore, that the raysdf,ef, arrive at the focus under the same convergency, and with the same light as if they proceeded from a lens of 6 inches diameter, placed at a distance beyond the object-glass CD (as GH,) determined by producing those rays till they meet the sides of the tube in GH, namely at 62.5 inches beyond the fluid lens. Hence, it isobvious, the rays will converge as they would do from an object-glass GH of the usual kind with a focus of 10 feet 5 inches. We have thus, therefore, shortened the tube 38.5 inches, or have at least the advantage of a focus 38.5 inches longer than our tube; and the same principle may be carried much farther, so as to reduce the usual length of refracting telescopes nearly one half without increasing the aberration in the first glass beyond the least that can possibly belong to a telescope of the usual kind of the whole length. It should likewise be observed that the adjustment for focus may be made either in the usual way, or by a slight movement of the fluid lens, as in the Gregorian Reflectors, by means of the small speculum.
Mr. Barlow afterwards constructed another and a larger telescope on the same principle, the clear aperture of which is 7.8 inches. Its tube is 11 feet, which, together with the eye-piece, makes the whole length 12 feet, but its effective focus is on the principle stated above, 18 feet. It carries a power of 700 on the closest double stars in South’s and Herschel’s catalogue, and the stars are, with that power, round and defined, although the field is not then so bright as could be desired. The telescope is mounted on a revolving stand, which works with considerable accuracy as an azimuth and altitude instrument. To give steadiness to the stand it has been made substantial and heavy; its weight by estimation being 400 pounds, and that of the telescope 130 pounds, yet its motions are so smooth, and the power so arranged, that it may be managed by one person with the greatest ease, the star being followed by a slight touch, scarcely exceeding that of the keys of a piano-forte. The focal length of the plate lens is78 inches, and of the fluid lens 59.8 inches—which at the distance of 40 inches produce a focal length of 104 inches, a total length of 12 feet, and an equivalent focus of 18 feet. The curves of the parallel meniscus checks for containing the fluid are—30 inches, and 144 inches, the latter towards the eye. The curves for the plate lens are 56.4 and 144. There is an interior tube 5 inches diameter, and 3 feet 6 inches long, which carries the cell in which the fluid is enclosed, and an apparatus by which it may be moved backwards and forwards, so that the proper adjustment may be made for colour, in the first instance, and afterwards the focus is obtained by the usual rack-work motion. The following is the mode by which the fluid was enclosed. After the best position has been determined practically for the checks forming the fluid lens, these, with the ring between them ground and polished accurately to the same curves, are applied together, and taken into an artificial high temperature, exceeding the greatest at which the telescope is ever expected to be used. After remaining here with the fluid some time, the space between the glasses is completely filled, immediately closed, cooled down by evaporation, and removed into a lower temperature. By this means a sudden condensation takes place, an external pressure is brought on the checks, and a bubble formed inside, which is of course filled with the vapour of the fluid; the excess of the atmospheric pressure beyond that of the vapour being afterwards always acting externally to prevent contact. The extreme edges are then sealed with the serum of human blood, or by strong fish-glue, and some thin pliable metal surface. By this process, Mr. Barlow says, ‘I have every reason to believe thelens becomes as durable as any lens of solid glass. At all events I have the satisfaction of stating, that my first 3 inch telescope has now been completed more than fifteen months, and that no change whatever has taken place in its performance, nor the least perceptible alteration either in the quantity or the quality of the fluid.’
The following are some of the observations which have been made with this telescope, and the tests to which it has been subjected. The very small star which accompanies the pole-star is generally one of the first tests applied to telescopes. This small point of light appeared brilliant and distinct; it was best seen with a power of 120, but was visible with a power of 700. The small star in Aldebaran was very distinct with a power of 120. The small star α Lyræ was distinctly visible with the same power. The small star called by Sir J. HerschelDebilissima, between 4 ε and 5 Lyræ, whose existence, he says, could not be suspected in either the 5 or 7 feet equatorial, and invisible also with the 7 and 10 feet reflectors of six and 9 inches aperture, but seen double with the 20 feet reflector, is seen very satisfactorily double with this telescope. η Persei, marked as double in South and Herschel’s catalogue, at the distance of 28´´, with another small star at the distance of 3´ 67´´, is seen distinctly sixfold, four of the small stars being within a considerably less distance than the remote one of η marked in the catalogue. And, rejecting the remote star, the principal, and the four other stars, form a miniature representation of Jupiter and his satellites, three of them being nearly in a line on one side, and the other on the opposite.Castor, is distinctly double with 120, and well opened and stars perfectly round with 360 and700: γ Leonis and α Piscium are seen with the same powers equally round and distinct. In ε Bootis, the small star is well separated from the larger, and its blue colour well marked with a power of 360. η Coronæ Borealis is seen double with a power of 360 and 700. 52 Orionis, ζ Orionis, and others of the same class are also well defined with the same powers. In regard to the planets which happened to be visible—Venus appeared beautifully white and well defined with a power of 120, but showed some colour with 360. Saturn with the 120 power, is a very brilliant object, the double ring and belts being well and satisfactorily defined, and with the 360 power, it is still very fine. The moon also is remarkably beautiful, the edges and the shadows being well marked, while the quantity of light is such as to bring to view every minute distinction of figure and shade.
The principal objections that may be made to this construction of a telescope are such as these:—Can the fluid be permanently secured? Will it preserve its transparency and other optical properties? Will it not act upon the surface of the glass and partially destroy it? &c. To such enquiries Mr. Barlow replies, that experience is the only test we have; our spirit levels, spirit thermometers, &c., show that some fluids at least may be preserved for many years, without experiencing any change, and without producing any in the appearance of the glass tubes containing them. But should any of these happen, except the last, nothing can be more simple than to supply the means of replacing the fluid at any time, and by any person, without disturbing the adjustment of the telescope. He expresses his hope that, should these experiments be prosecuted, an achromatic telescope may ultimately be produced whichshall exceed in aperture and power, any instruments of the kind hitherto attempted. If the prejudice against the use of fluids could be removed, he feels convinced that well-directed practice would soon lead to the construction of the most perfect instruments, on this principle, at a comparatively small expense. ‘I am convinced,’ he says, ‘judging from what has been paid for large object-glasses, that my telescope, telescope stand, and the building for observation, with every other requisite convenience, have been constructed for a less sum than would be demanded for the object-glass only, if one could be produced of the same diameter of plate and flint-glass; and this is a consideration which should have some weight, and encourage a perseverance in the principle of construction.’24
The object of this construction is to render a small disc of flint-glass available to perform the office of compensation to a much larger one of crown-glass, and thus to render possible the construction of telescopes of much larger aperture than are now common, without hindrance from the difficulty at present experienced in procuring large discs of flint-glass. It is well known tothose who are acquainted with telescopes, that in the construction of an ordinary achromatic object-glass, in which a single crown lens is compensated by a single one of flint, the two lenses admit of being separated only by an interval too small to afford any material advantage,in diminishing the diameter of the flint lens, by placing it in a narrower part of the cone of rays—the actual amount of their difference in point of dispersive power being such as to render the correction of the chromatic aberration impossible, when their mutual distance exceeds a certain limit. This inconvenience Mr. Rogers proposes to obviate, by employing, as a correcting lens—not a single lens of flint, but a compound one consisting of a convex crown and concave flint, whose foci are such as to cause their combination to act as a plain glass on the mean refrangible rays. Then it is evident, that by means of the greater dispersive power of flint than of crown glass, this will act as a concave on the violet, and as a convex on the red rays, andthatthe more powerfully, according as the lenses separately have greater powers or curvature. If then, such a compound lens be interposed between the object-glass of a telescope—supposed to be a single lens of plate or crown-glass—and its focus, it will cause no alteration in the focus for mean rays, while it will lengthen the focus for violet, and shorten it for red rays. Now this is precisely what is wanted to produce an achromatic union of all the rays in the focus; and as nothing in this construction limits the powers of the individual correcting lenses, they may therefore be applied any where that convenience may dictate; and thus, theoretically speaking, a disc of flint-glass, however small, may be made to correct the colour of one of crown however large.
This construction, likewise, possesses other and very remarkable advantages. For, first, when the correcting lens is approximately constructed on a calculation founded on its intended aperture, and on the refractive and dispersive indices of its materials, the final and complete dispersion of colour may be effected, not by altering the lenses by grinding them anew, but by shifting the combination nearer to, or farther from, the object-glass, as occasion may require, along the tube of a telescope, by a screw motion, till the condition of achromaticity is satisfied in the best manner possible. And secondly, the spherical aberration may in like manner be finally corrected, by slightly separating the lenses of the correcting glass, whose surfaces should for this purpose be figured to curvatures previously determined by calculation, to admit of this mode of correction—a condition which Mr. Rogers finds to be always possible. The following is the rule he lays down for the determination of the foci of the lenses of the correcting glass:—‘The focal length of either lens of the correcting lens is to that of the object-glass, in a ratio compounded of the ratio of the square of the aperture of the correcting lens to that of the object-glass, and of the ratio of the difference of the dispersive indices of the crown and flint glass, to the dispersive index of crown.’ For example, to correct the colour of a lens of crown or plate glass of 9 inches aperture, and 14 feet focal length (the dimensions of the telescope of Fraunhofer at Dorpat) by a disc of flint glass 3 inches in diameter, the focus of either lens of the correcting lens will require to be about 9 inches. To correct it by a 4 inch disc will require a focus of about 16 inches each.
Mr. Rogers remarks, that it is not indispensableto make the correcting glass act as a plane lens. It is sufficient if it be so adjusted as to have a shorter focus for red rays than for violet. If, preserving this condition, it be made to act as a concave lens, the advantage procured by Mr. Barlow’s construction of reducing the length of the telescope with the same focal power, is secured, and he considers, moreover, that by a proper adaptation of the distances, foci, &c., of the lenses, we might hope to combine with all these advantages that of the destruction of the secondary spectrum, and thus obtain a perfect telescope.
The above is an abstract of a paper read to the ‘Astronomical Society of London’ in April 1828, by A. Rogers, Esq.
The reader will easily perceive that the principle on which Mr. Rogers proposes to construct his telescope is very nearly similar to that of professor Barlow, described above, with this difference, that the correcting lens of the Professor’s telescope is composed of a transparentfluid, while that of Mr. Rogers is asolidlens consisting of a convex crown and concave flint. The general object intended to be accomplished by both is the same, namely, to make a correcting lens of a comparatively small diameter serve the purpose of a large disc of flint glass, which has hitherto been very expensive, and very difficult to be procured; and likewise to reduce the length of the telescope while the advantage of a long focal power is secured.—A telescope, on this principle, was constructed 7 or 8 years ago by Mr. Wilson, lecturer on Philosophy and Chemistry, Glasgow, before he was aware that Mr. Rogers had proposed a similar plan. I have had an opportunity of particularly inspecting Mr. Wilson’s telescope, and trying its effects on terrestrial objects with high powers, andwas on the whole highly pleased with its performance. It appeared to be almost perfectly achromatic, and produced a distinct andwell-definedimage of minute distant objects, such as small letters on sign-posts, at 2, 3 and 4 miles distant. But I had no opportunity of trying its effects on double stars or any other celestial objects. The instrument is above 6 feet long; the object lens is a plano-convex of crown glass 4 feet focal distance, and 4 inches diameter, the plain side next the object.
At 26 inches distant from the object lens is the compound lens of 2 inches in diameter; and the two lenses of which it is composed are both ground to a radius of 3¾ inches. That made of crown glass isplano-convex, the other, made of flint glass, is plano-concave, and are placed close together, the convex side being next the object, and the concave side next the eye. The greater refractive power of the flint glass renders the compound one slightly concave in its effect (although the radius of curvature is similar in both), and lengthens the focus to 6 feet from the object-glass; and this is consequently the length of the instrument. The compound corrector so placed intercepts all those rays which go to form the image in the field of view, producing there an achromatic image. The concave power of the corrector renders the image larger than if directly produced by a convex lens of the same focus. The concavity of the corrector is valuable also in this respect, that a very slight alteration in its distance from the object-glass, changes the focal distance much more than if it were plain, and enables us to adjust the instrument to perfect achromatism with great precision.
SECT. 1.—HISTORY OF THE INVENTION, AND A GENERAL DESCRIPTION OF THE CONSTRUCTION OF THESE INSTRUMENTS.
Reflecting telescopes are those which represent the images of distant objects by reflection, chiefly from concave mirrors.
Before the achromatic telescope was invented, there were two glaring imperfections in refracting telescopes, which the astronomers of the 17th century were anxious to correct. The first was its very great length when a high power was to be applied, which rendered it very unwieldy and difficult to use. The second imperfection was the incorrectness of the image as formed by a single lens. Mathematicians had demonstrated that a pencil of rays could not be collected in a single point by a spherical lens, and also that the image transmitted by such a lens would be in some degree incurvated. After several attempts had been made to correct this imperfection by grinding lenses to the figure of one of the conic sections, Sir I. Newton happened to commence an examination of the colours formed by a prism; and having,by the means of this simple instrument, discovered the different refrangibility of the rays of light—to which we have several times adverted in the preceding descriptions—he then perceived that the errors of telescopes, arising from that cause alone, were some hundred times greater than such as were occasioned by the spherical figure of lenses; which induced this illustrious philosopher to turn his attention to the improvement of telescopes by reflection.
It is generally supposed that Mr. James Gregory—a son of the Rev. John Gregory, minister of Drumoak in the county of Aberdeen—was the first who suggested the construction of a reflecting telescope. He was a young man of uncommon genius, and an eminent mathematician; and in the year 1663, at the age of only 24, he published in London, his treatise entitled ‘Optica Promota,’ in which he explained the theory of that species of reflecting telescope which still bears his name, and which he stated as being his own invention. But as Gregory, according to his own account, was endowed with no mechanical dexterity, and could find no workman capable of realizing his invention—after some fruitless attempts to form proper specula, he was obliged to give up the pursuit; so that this telescope remained for a considerable time neglected. It was several years after Gregory suggested the construction of reflecting telescopes, till Newton directed his attention fully to the subject. In a letter addressed to the secretary of the Royal Society, dated in February, 1672, he says, ‘Finding reflections to be regular, so that the angle of reflection of all sorts of rays was equal to the angle of incidence, I understood that, by their mediation, optic instruments might be brought toany degree of perfection imaginable, providing a reflecting substance could be found which would polish as finely as glass, and reflect as much light as glass transmits, and the art of communicating to it a parabolic figure be also obtained. Amidst these thoughts I was forced from Cambridge by the intervening plague, and it was more than two years before I proceeded further.’
It was towards the end of 1668, or in the beginning of the following year, when Newton, being obliged to have recourse to reflectors, and not relying on any artificer for making the specula, set about the work himself, and early in the year 1672, completed two small reflecting telescopes. In these he ground the great speculum into a spherical concave, although he approved of the parabolic form, but found himself unable to accomplish it. These telescopes were of a construction somewhat different from what Gregory had suggested, and though only 6 inches long, were considered as equal to a 6 feet common refracting telescope. It is not a little singular, however, that we hear no more about the construction of reflectors till more than half a century afterwards. It was not till the year 1723, that any reflectors were known to have been made, adapted to celestial observations. In that year, Mr. Hadley, the inventor of the reflecting quadrant, which goes by his name, published in No. 376 of the Philosophical Transactions, an account of a large reflector on Newton’s plan, which he had just then constructed, the performance of which left no room to doubt that this invention would remain any longer in obscurity. The large speculum of this instrument was 62â… inches focal distance and 5 inches diameter, was furnished with magnifying powers of from 190 to 230 times,and equalled in performance the famous aerial telescope of Huygens of 123 feet in length.25Since this period, the reflecting telescope has been in general use among astronomers in most countries of Europe, and has received numerous improvements, under the direction of Short, Mudge, Edwards and Herschel—the last of whom constructed reflectors of 7, 10, 20, and even 40 feet in focal length, which far surpassed, in brightness and magnifying power, all the instruments of this description, which had previously been attempted.
I shall now proceed to give a brief sketch of the nature of a reflecting telescope, and the different forms in which they have been proposed to be constructed.
Fig. 62 represents the reflecting telescope as originally proposed by Gregory. ABEF represents a tube open at AF towards the object; at the other end is placed a concave speculum BE, with a hole CD in its centre, the focus of which is ate. A little beyond this focus, towards the object end of the telescope AF, is placed another small concave mirror G, having its polished face turned towards the great speculum, and is supported by an arm GH fastened to a slider connected with the tube. At the end of the great tube BE is screwed in a small tube CDKI, containing a small plano-convex lens IK. Such are the essential parts of this instrument and their relative positions. It will be recollected in our description of the properties of concave mirrors (see page 92), that, when rays proceed from adistant object, and fall upon a concave-speculum, they paint an image or representation of the object in its focus before the speculum. Now suppose two parallel raysabfalling on the speculum BE, incd; they are reflected to its focusewhere an inverted image of the object is formed. This image is formed at a little more than the focal distance of the small speculum from its surface, and serves as it were for an object on which the small mirror may act. By the action of this mirror this first image is reflected to a point aboutf, where a second image is formed very large and erect. This image is magnified in the proportion offG toeG, the rays from which are transmitted to the eye glass IK, through which the eye perceives the object clear and distinct, after the proper adjustments have been made.
Suppose the focal distance of the great mirror was 9 inches, and the focal distance of the small mirror 1½ inch—were we to remove the eye piece of this telescope, and look through the hole of the great mirror, we should see the image of the object depicted upon the face of the small speculum, and magnified, in the proportion of 9 to 1½, or, 6 times, on the same principle as a common convex object glass 9 inches focal length, with an eye glass whose focus is 1½ inch magnifies 6 times. This may be regarded as the first part of the magnifying power. If now, we suppose the small speculum placed a little more than 1½ inch from the image formed by the great speculum, a second image is formed aboutf, as much exceeding the first in its dimensions as it exceeds it in distance from the small speculum, on the principle on which the object glass of a compound microscope forms a large image near the eye glass. Suppose this distance to be 9 times greater, then the whole magnifying power will be compounded of 6 multiplied by 9, or 54 times. As a telescope it magnifies 6 times, and in the microscope part 9 times.—Such is ageneralidea of the Gregorian telescope, the minute particulars and structure of which can only be clearly perceived by a direct inspection of the instrument.
The Newtonian Reflector.—This instrument is somewhat different both in its form and in its mode of operation from that of Gregory. It is represented in fig. 63, where BAEF is the tube, and BE, the object concave mirror, which reflects the parallel raysabto aplanespeculum G, placed 45°, or half a right angle to the axis of the concave speculum. This small plane reflector must be of an oval form, the length of the oval should be to the breadth as 7 to 5, on account of the obliquityof its position. It is supported on an arm fixed to the side of the tube; an eye-glass is placed in a small tube, moveable in the larger tube, so as to be perpendicular to the axis of the large reflector, the perpendicular line passing through the centre of the small mirror. The small mirror is situated between the large mirror and its focus, that its distance from this focal point may be equal to the distance from the centre of the mirror to the focus of the eye-glass. When the raysabfrom a distant object fall upon the large speculum atcd, they are reflected towards a focus ath; but being intercepted by the plane mirror G, they are reflected perpendicularly to the eye-glass at I, in the side of the tube, and the image formed near that position ateis viewed through a small plano-convex lens. The magnifying power of this telescope is in the proportion of the focal distance of the speculum to that of the eye-glass. Thus, if the focal distance of the speculum be 36 inches, and that of the eye-glass1/3of an inch, the magnifying power will be 108 times. It was this form of the reflecting telescope, that Newton invented, which Sir. W. Herschel adopted, and with which he made most of his observations and discoveries.
The Cassegrainian Reflector.—This mode of the reflecting telescope, suggested by M. Cassegrain, a Frenchman, is represented in fig. 64. It is constructed in the same way as the Gregorian, with the exception of a smallconvexspeculum G being substituted in the room of the small concave in Gregory’s construction. As the focus of a convex mirror is negative, it is placed at a distance from the large speculum equal to the difference of their foci, that is, if the focal length of the large speculum be 18 inches, and that of the small convex 2 inches, they are placed at 16inches distant from each other, on a principle similar to that of the Galilean telescope, in which the concave eye-glass is placed within the focus of the object-glass by a space equal to the focal length of the eye-glass. In this telescope, likewise, instead of two there is onlyone imageformed, namely that in the focus of the eye-glass; and, on this account some are of opinion that the distinctness is considerably greater than in the Gregorian. Mr. Ramsden was of opinion that this construction is preferable to either of the former reflectors, because the aberrations of the two metals have a tendency to correct each other, whereas in the Gregorian both the metals being concave, any error in the specula will be doubled. It is his opinion that the aberrations in the Cassegrainian construction to that of the Gregorian is as 3 to 5. The length of this telescope is shorter than that of a Gregorian of equal focal length, by twice the focal length of the small mirror, and it shows every thing in aninvertedposition, and consequently is not adapted for viewing terrestrial objects.
Dr. Hook’s Reflector.—Before the reflecting telescope was much known, Dr. Hook contrived one, the form of which is represented, fig. 65, which differs in little or nothing from the Gregorian, except that the eye-glass I is placed in the hole of the great speculum BE.
Martin’s Reflector.—Mr. Bengamin Martin, a distinguished writer on optical and philosophical science, about a century ago, described a new form of the reflecting telescope, approximating to the Newtonian structure, which he contrived for his own use. It is represented in fig. 66. ABEF is the tube, in which there is an opening or aperture OP, in the upper part. Against this holewithin the tube is placed a large plane speculum GH, at half a right angle with the axis or sides of the tubes, with a hole CD perforated through its middle. The parallel raysa bfalling on the inclined plane GH are reflected perpendicularly and parallel on the great speculum BE in the bottom of the tube. From thence they are reflected converging to a focusethrough the hole of the plane mirror CD, which being also the focus of the eye-glass IK, the eye will perceive the object magnified and distinct.
In the figures referred to in the above descriptions, only one eye-glass is represented to avoid complexity; but in most reflecting telescopes, the eye-piece consists of a combination of two plano-convex glasses, as in fig. 67, which produces a more correct and a larger field of view than a single lens. This combination is generally known by the name of theHuygenian eye-piecewhich shall be described in the section on theeye-piecesof telescopes.
The following rule has been given for finding the magnifying power of the Gregorian telescope:—Multiply the focal distance of the great mirror by the distance of the small mirror from the image next the eye; and multiply the focal distance of the small mirror by the focal distance of the eye-glass; then divide the product of the former multiplication by the product of the latter, and the quotient will express the magnifying power. The following are the dimensions of one of the reflecting telescopes constructed by Mr. Short—who was long distinguished as the most eminent maker of such instruments, on a large scale, and whose large reflectors are still to be found in various observatories throughout Europe.
The focal distance of the great mirror 9.6 inches; or Pm, fig. 67, its breadth FD 2.3; thefocal distance of the small mirror Ln1.5—or 1½ inch—its breadthg h0.6—or6/10of an inch; the breadth of the hole in the great mirror UV, 0.5—or half an inch—the distance between the small mirror and the next eye-glass LR, 14.2; the distance between the two eye-glasses SR, 2.4; the focal distance of the eye-glass next the metal, 3.8.; and the focal distance of the eye-glass next the eye, Sa1.1, or one inch and one tenth. The magnifying power of this telescope was about 60 times. Taking this telescope as a standard, the following table of the dimensions and magnifying powers of Gregorian reflecting telescopes, as constructed by Mr. Short, has been computed.