Rotation of Plane of Polarisation.

Fig. 176.—Axis of Crystals of Iceland Spar.

Fig. 177.—A Rhomb showing the passage of Rays of Light.

Mr. Nicol, of Edinburgh first succeeded in making a rhomb of Iceland spar into asingle-image prism. His method of splitting up the crystal into two equal parts was as follows:—

A rhomb of Iceland spar of one-fourth of an inch in length, and about four-eighths of an inch in breadth and thickness, is divided into two equal portions in a plane, passing through the acute lateral angle, and nearly touching the obtuse side angle. The sectional plane of each of these halves must be carefully polished, and the two portions cemented firmly together with Canada balsam, so as to form a rhomb similar to that before division; by this management the ordinary and extraordinary rays are so separated that only one is transmitted: the cause of this greatdivergence of the rays is considered to be owing to the action of the Canada balsam, the refractive index of which (1·549) is that between the ordinary (1·6543) and the extraordinary (1·4833) refraction of calcareous spar, and which will change the direction of both rays in an opposite manner before they enter the posterior half of the combination. The direction of rays passing through such a prism is indicated by the arrow,Fig. 178.

Fig. 178.

Polarised light cannot be distinguished from common light, as already said, by the naked eye; and for all experimental purposes in polarisation, two pieces of apparatus must be employed, one to produce polarisation, and the other to show or an analyse it. The former is called thepolariser, the latter theanalyser; and every apparatus that serves for one of these purposes will also serve for the other.

Fig. 179.—Polariser.Fig. 179a.—Analyser.

Fig. 179.—Polariser.

Fig. 179a.—Analyser.

Polarising Apparatus for Students’ Microscope.

In all cases there are two positions, differing by 180°, which give a minimum of light, and the two positions intermediate between these give a maximum of light. The extent of the changes thus observed is a measure of the completeness of the polarisation of light.

The two prisms mounted as shown in Figs. 179 and 179aconstitute the apparatus adapted to the microscope. The polariser slips into place below the stage, and the analyser, with the prism fixed in a tube, is screwed in above the objective.

The definition is considered by some experimenters as somewhat better if the analyser be used above the eye-piece, and is certainly more easily rotated.

Fig. 180.—Prism mounted as an Eye-piece.

Method of employing the Polarising Prism(Fig. 179).—After having adapted it to slide into a groove on the under-surface of the stage, where it is secured and kept in place by the small milled-head screw, the other prismFig. 179a) is screwed on above the object-glass, and thus passes directly into the body of the microscope. The light from the mirror having been reflected through them the axes of the two prisms must be made to coincide; this is done by regulating the milled-head screw until, by revolving thepolarisingprism, the field of view is entirely darkened twice during its revolution. If very minute salts or crystals are submitted for examination then it will be found preferable to place the analyser above the eye-piece, as inFig. 180. Thus thepolariscopeis seen to consist of two parts; one forpolarising, the other foranalysingor testing the light. There is no essential difference between the two parts, except what convenience or economy may lead us to adopt; and either part, therefore, may be used as polariser or analyser; but whichever is used as the polariser, the other becomes the analyser.

Fig. 181.—More Modern Polariser and Analyser.

Opticians have their own methods of adapting the polariser and analyser to their several microscopes. Watson’s special form of apparatus is represented inFig. 181, the polariser being adapted to the sub-stage, and the analyser to screw into the objective.

Tourmaline.—A semi-transparent mineral, of a neutral or bluish tint, called tourmaline, when cut into thin slices (about1⁄20-inch thick) with their faces parallel to their axes exhibit the same phenomena as the Nicol prism. The only objection to which is that the transmitted polarised beam is more or less coloured. The tourmaline to be preferred stops the most light when its axis is at right-angles to that of the polariser, and yet admits the most when in the same plane. Make choice of a tourmaline as perfect as possible; size is of less importance when intended for use with the microscope.

Transmission of rays through tourmaline is only one of several ways in which light can be polarised. When a beam of light is reflected from a polished surface of glass, wood, ivory, leather, or any other non-metallic substance, at an angle of 50° to 60° with the normal, it is more or less polarised, and in like manner a reflector composed of any of these substances may be employed as an analyser. In so using it, it should be rotated about an axis parallel to the incident rays which are to be tested, and the observation consists in noting whether this rotation produces changes in the amount of reflected light.

For every reflected substance there is a particular angle of incidence, which gives a maximum of polarisation in reflected light. It is called thepolarising anglefor the substance, and its tangent is always equal to the index of refraction of the substance; or, what amounts to the same thing, it is that particular angle of incidence which is the complement of the angle of refraction, so that the refracted rays are at right angles. This important law was discovered experimentally by Sir David Brewster.

Tourmaline, like Iceland spar, is a negative uniaxial crystal; and its use as a polariser depends on the property which it possesses of absorbing the ordinary much more rapidly than the extraordinary ray, so that a thickness which is tolerably transparent to the latter is almost completely opaque to the former. Its pale cobalt blue colour enhances the beauty of certain crystal and mineral substances, but like Iceland spar, the paler and more perfect crystals are becoming scarce.

Seleniteis another mineral of value in polarisation experiments. It is a native crystalline hydrated sulphate of lime. A beautiful fibrous variety calledsatin-gypsumis found in Derbyshire. Theform of the crystal most frequently met with is that of an oblique rectangular prism, with ten rhomboidal faces, two of which are much larger than the rest. It is usually split up into thin laminæ parallel to their lateral faces; each film should have a thickness of from one-twentieth to one-sixtieth of an inch. In the two rectangular directions these films allow perpendicular rays of polarised light to traverse them unchanged, termed theirneutral axes. In two other directions, however, which form respectively angles of 45° with the neutral axes, these films have the property of double refraction, a direction known as thedepolarising axis.

Fig. 182.—Darker’s Selenite Films and Stage.

The thickness of the film of selenite determines the particular tint. If, therefore, we use a film of irregular thickness, different colours are presented by the different thicknesses. These facts admit of very curious and beautiful illustration, when used under the object placed on the stage of the microscope. The films employed should be mounted between two glasses for protection. Some persons employ a large film, mounted in this way between the plates of glass, with a raised edge, to act as a stage for supporting the object, it is then called the “selenite stage.” The best film for the microscope is that which gives blue, and its complementary colour yellow. The late Mr. Darker constructed a selenite stage for the purpose (Fig. 182). With this a mixture of colours will be brought about, by superimposing three films, one on the other. By slight variations in their positions, produced by means of an endless-screw motion, all the colours of the spectrum can be shown. When objects are thus exhibited, it should be borne in mind that all negative tints, as they are termed, are diminished, and all positive tints increased; the effect of which is to mask the true character of the phenomena.

For a certain thickness of selenite the ellipse will become a circle, and we have thus what is calledcircularly-polarisedlight, which is characterised by the property that rotation of the analyser producesno change of intensity. Circularly-polarised light is not, however, identical with ordinary light; for the interposition of an additional thickness of selenite converts it into elliptically (or in a particular case into plane) polarised light.

It is necessary, for the exhibition of colour in our experiments, that the plate of selenite should be very thin, otherwise the retardation of one component vibration as compared with the other will be greater by several complete periods for violet than for red, so that the ellipses will be identical for several different colours, and the total non-suppressed light will be sensibly white in all positions of the analyser.

Two thick plates may, however, be so combined as to produce the effect of one thin plate. For example, two selenite plates of nearly equal thickness may be laid one upon the other, so that the direction of greatest elasticity in the one shall be parallel to that of least elasticity in the other. The resultant effect in this case will be that due to the difference of their thicknesses. Two plates so laid are said to becrossed.

Fig. 183.—Redis represented by perpendicular lines;Greenby oblique.

The following experiments will well serve to illustrate some of the more striking phenomena of double refraction, and will also be a useful introduction to its practical application. Take a plate of brass (Fig. 183) three inches by one, perforated with a series of holes from about one-sixteenth to one-fourth of an inch in diameter; the size of the smallest should be in accordance with the power of the objective, and the separating power of the double refraction.

Experiment1.—Place the brass plate so that the smallest hole shall be in the centre of the stage of the microscope; employ a low power (1½ or 2 inches) objective, and adjust the focus as for the ordinary microscopic object; place the double image prism over the eye-piece, and two distinct images will be seen; by revolving the prism, the images will describe a circle, the circumference of which will cut the centre of the field of view; one of which is the ordinary,the other the extraordinary ray. By moving the slide from left to right the larger orifices will appear in the field, the images seen will not be completely separated, but will overlap, as represented in the figure.

Experiment2.—Insert the Nicol’s prism into its place under the stage, still retaining the double image prism over the eye-piece; then, by examining the object, there will appear in some positions two images, in others only one image; it will be seen, that at 90° this ray will be cut off, and that which was first observed will become visible; at 180°, or one-half the circle, an alternate change will take place; at 270°, another change; and at 360°, the completion of the circle, the first image will reappear.

Before proceeding to make the next experiment, the position of the Nicol’s prism should be adjusted, and its angles brought parallel with the square of the stage. The true relative position of the selenite should also be determined by noticing the natural flaws in the film, which should run parallel with each other, and be adjusted at an angle of about 46° with the square bars of the stage.

Experiment3.—If we now take the plate of selenite thus prepared, and place it under the piece of brass on the stage, we shall see, instead of the alternate black and white images, two coloured images composed of the constituents of white light, which will alternately change by revolving the eye-piece at every quarter of the circle; then, by passing along the brass, the images will overlap; and at the point at which they do so, white light will be produced. If, by accident, the prism be placed at an angle of 45° from the square part of the stage, no particular colour will be perceived, and it will then illustrate the phenomena of the neutral axis of the selenite, because when placed in the relative position no depolarisation takes place. The phenomena of polarised light may be further illustrated by the addition of a second double image prism, and a film of selenite adapted between the two. The systems of coloured rings in crystals cut perpendicularly to the principal axis of the crystal are best seen by employing the lowest object-glass.

Biaxial Crystals.—To show perfectly the beautiful series ofrings and brusheswhich biaxial crystals exhibit, it becomes necessary to convert the microscope, for the time being, into (so to speak) a wide-angled telescope.

Huyghenian Eye-piece.Inner draw-tube.Objective in draw-tube.Analysing Prism.Objective.Specimen under Examination.Sub-stage Condenser.Polarising Prism, fixed in sub-stage below.Fig. 184.—Diagrammatic arrangement of the Polarising Microscope.

Huyghenian Eye-piece.Inner draw-tube.Objective in draw-tube.Analysing Prism.Objective.Specimen under Examination.Sub-stage Condenser.Polarising Prism, fixed in sub-stage below.

Huyghenian Eye-piece.

Inner draw-tube.

Objective in draw-tube.

Analysing Prism.

Objective.

Specimen under Examination.

Sub-stage Condenser.

Polarising Prism, fixed in sub-stage below.

Fig. 184.—Diagrammatic arrangement of the Polarising Microscope.

In Sub-stage: P, polarising prism; C, sub-stage condenser on stage; M, mineral or crystal. On nose-piece: O1, objective,4⁄10-inch; A, analysing prism.

In Draw-tube: O2, 2 or 3 inch Objective; H, Huyghenian eye-piece.

For the purpose, screw on a low-power objective to the end of the draw-tube (Fig. 184).31As the light requires to be passed through the crystals at a considerable angle, a wide-angled condenser should be employed, but it need not be achromatic. The objective most suitable is a4⁄10-inch, of ·64 numerical aperture, but a ¼-inchof ·71 numerical aperture, or a1⁄3-inch of ·65 numerical aperture, will answer the purpose equally well. As the whole of the back lens of the objective should be visible through the analysing Nicol prism, the back lens of the objective must not be too large; thus a ½-inch of ·65 numerical aperture will not be so effective. The analysing prism may be placed either where it is in the drawing, below the stage, or above the eye-piece. It works equally well above the objective, the position it ordinarily occupies in the microscope.

For the draw-tube a 2-inch objective and a B Huyghenian eye-piece answers very well. Before screwing the objective on to the end of the draw-tube centre the light in the usual manner, the Nicol’s being turned so as to give a light field, then screw the objective on to the end of the aperture, and put the crystal on the stage, rack down the body so that the objective on the nose-piece nearly touches the crystal, then focus with the draw-tube only. The sub-stage condenser should be racked up close to the underside of the crystal.

Opticians, however, have more recently furnished a special form of microscope (The Petrological Microscope,Fig. 79, p. 112), for the use of those students who may desire to prosecute so fascinating a study, and determine the optic axial angles of crystals.

Fuess32lately introduced a new form of microscope for polarising and viewing biaxial crystals, which he believes to be needed, as in the ordinary microscope the opening of the polariser is scarcely a third of that of the condenser; moreover, it is not absolutely necessary that the polariser and analyser should be Nicol’s prisms. This fact was discovered by myself many years ago. Fuess utilises a bundle of thin glass plates, as in the older Nuremberg polariscope. The frame holding plates can be readily adjusted at the proper polarising angle, the analyser being the ordinary small Nicol, screwed above the objective. The illuminator is an Abbe’s triple condenser, of numerical aperture 1·40, which can be adjusted in the ordinary way. The front lens of this should have a diameter of 11·12 mm. and the lower lens of 30 mm. This increase in the condenser fully compensates for the loss of light by the bundle of glass plates, and also enables thick sections of crystals to be examined in convergent polarised light. The ocular used should have a large field; the A Huyghenian answers best. A suggestion to return to the originalNuremberg polariser is very opportune, asIceland spar is becoming scarce.

Mr. A. Mickel accidentally discovered that an opalescent mirror can be converted into an excellent and inexpensive substitute for the Nicol-prism polariser.

When a plate of quartz (rock-crystal), even of considerable thickness, cut perpendicular to the axis, is interposed between the polariser and analyser, colour is exhibited, the tints changing as the analyser is rotated; and similar effects of colour are produced by employing, instead of quartz, a solution of sugar enclosed in a tube with plain glass ends.

The action thus exerted by quartz and sugar is calledrotation of the plane of polarisation, a name which sufficiently expresses the observed phenomena. In the case of ordinary quartz, and solutions of sugar-candy, it is necessary to rotate the analyser in the direction of watch-hands as seen by the observer, and the rotation of the plane of polarisation is said to beright-handed. In the case of what is calledleft-handedquartz, and of solutions of non-crystallisable sugar, the rotation of the plane of polarisation is in the opposite direction, and the observer must rotate the analyser against watch-hands.

Quartzbelongs to the uniaxial system of crystals, and accordingly exhibits one series of rings only, and no perfect central black cross.

On revolving the tourmaline the colour gradually changes, and passes through all the colours of the spectrum. It can be cut to exhibit either right-handed polarisation or left-handed polarisation and also to exhibit straight lines.

Calc Spar.—A uniaxial crystal showing only one system of rings, and a black cross, changing into a white cross on revolving the tourmaline.

Topaz.—A biaxial crystal exhibiting only one system of rings with one fringe, owing to the wide separation of the axes. The fringe and colours change on revolving the tourmaline.

Borax.—A biaxial crystal; the colours are seen to be more intense than in topaz, but the rings not so complete—only one set of rings can be seen, owing to their wide separation.

Rochelle Salt.—A biaxial crystal; the colours are more widely spread out than the former, and only one set of rings seen at the same time.

Carbonate of Lead.—A biaxial crystal; axes not so far separated, and both systems of rings are more widely spread than those of potassium nitrate.

Aragonite.—A biaxial crystal; axes widely separated, but both systems of rings seen at the same time. A fine crystal for displaying the biaxial system.

Fig. 185.—Crystal of Potassium Nitrate.

It was long believed that all crystals had only one axis of double refraction; but Brewster found that the greater number of crystals which occur in the mineral kingdom havetwo axesof double refraction, or rather axes around which double refraction takes place; in the axes themselves there is no double refraction.

Potassium nitrate crystallises in six-sided prisms with angles of about 120°. It has two axes of double refraction. These axes are each inclined about 2½° to the axes of the prism, and 5° to each other. If, therefore, a small piece be split off a prism of potassium nitrate with a knife driven by a sharp blow of a hammer, and the two surfaces polished perpendicular to the axes of the prism, so as to leave the thickness of the sixth or eighth of an inch, and then a ray of polarised light be transmitted along the axes of the prism, the double system of rings will be clearly visible.

When the line connecting the two axes of the crystal is inclined 45° to the plane of primitive polarisation, a cross is seen on revolving the potassium nitrate; it gradually assumes the form of two hyperbolic curves, as inFig. 185. But if the tourmaline beagain revolved through half a quadrant, the black cross will be replaced by white spaces, as in the second figure. These systems of rings have, generally speaking, the same colours as those of thin plates, or as those of a system of rings revolving around one axis. The orders of the colours commence at the centres of each system; but at a certain distance, which corresponds to the sixth ring, the rings, instead of returning and encircling each pole, encircle the two poles as an ellipse does its two foci. If the thickness of the plate ofnitrebe diminished or increased, the rings are diminished or increased according to the thickness of the crystal.

Small specimens of various salts may be crystallised and mounted in Canada balsam for viewing under the stage of the microscope; by arresting crystallisation at certain stages, a greater variety of forms and colours will be obtained: we may enumerate salicine, asparagine, acetate of copper, phospho-borate of soda, sugar, carbonate of lime, potassium chlorate, oxalic acid, and all the oxalates found in urine, with the other salts from the same fluid, a few of which are shown inPlate VIII.

The late Dr. Herapath described a salt of quinine, remarkable for its polarising properties. The crystals of this salt, when examined by reflected light, have a brilliant emerald-green colour, with almost a metallic lustre; they appear like portions of the elytræ of the cantharides beetle, and are also very similar to murexide in appearance. When examined by transmitted light, they scarcely possess any colour, there is only a slightly olive-green tinge; but if two crystals, crossing at right-angles, be examined, the spot where they intersect appears perfectly black, even if the crystals are not more than one five-hundredth of an inch in thickness. If the light be in the slightest degree polarised—as by reflection from a cloud, or by the blue sky, or from the glass surface of the mirror of the microscope placed at the polarising angle 65° 45′—these little prisms and films assume complementary colours: one appears green, and the other pink, and the part at which they cross is chocolate or deep chestnut-brown, instead of black. Dr. Herapath succeeded in making artificial tourmalines large enough to surmount the eye-piece of the microscope; so that all experiments with those crystals upon polarised light may be made without the tourmaline or Nicol’s prism. The finest rosette crystals are made as follows:—To a moderately strongsolution ofCinchonidineadd a drop or two of Herapath’s test-fluid.33A few drops of this is placed on the centre of a glass slide, and put aside until the first crystals are observed to be formed near the margin. The slide should now be placed upon the stage of the microscope, and the progress of formation of the crystals closely watched. When these are seen to be large enough, and it is deemed necessary to stop their further development, the slide must be quickly transferred to the palm of the hand, the warmth of which will be found sufficient to stop further crystallisation. These crystals attract moisture, deliquesce, and should therefore be kept in a perfectly dry place.

Fig. 186.—In this figure heraldic lines are adopted to denote colour. The dotted parts indicateyellow, the straight linesred, the horizontal linesblue, and the diagonal, or oblique lines,green. The arrows show the plane of the tourmaline,a, blue stage;b, red stage of selenite employed.

To render these crystals evident, it merely remains to bring the glass-slide upon the field of the microscope, with the selenite stage and single tourmaline, or Nicol’s prism, beneath it; instantly the crystals assume the two complementary colours of the stage: red and green, supposing that the pink stage is employed; or blue and yellow, provided the blue selenite is made use of. All those crystalsat right angles to the plane of the tourmaline produce that tint which an analysing-plate of tourmaline would produce when at right angles to the polarising-plate; whilst those at 90° to these educe the complementary tint, as the analysing-plate would also have done if revolved through an arc of 90°.

This test is a delicate one for quinine (Fig. 186,aandb); not only do these peculiar crystals act in the way just related, but they may be easily proved to possess the optical properties of that remarkable salt, the sulphate of iodo-quinine.

Fig. 187.—Polarised Crystals of Quinidine.

To test for quinidine, it is merely necessary to allow a drop of acid solution to evaporate to dryness upon the slide, and to examine the crystalline mass by two tourmalines, crossed at right angles, and without the stage. Immediately little circular discs of white, with a well-defined black cross, start into existence, should quinidine be present even in very minute traces. These crystals are represented inFig. 187.

If the selenite stage be employed in the examination of this object, one of the most gorgeous appearances in the whole domain of the polarising microscope is displayed: the black cross disappears, and is replaced by one consisting of two colours, and divided into a cross having a red and green fringe, whilst the four intermediate sectors are a gorgeous orange-yellow. These appearances alteron the revolution of the analysing-plate of tourmaline; when the blue stage is employed, the cross assumes a blue or yellow tint, varying according to the position of the analysing plate. These phenomena are analogous to those exhibited by certain circular crystals of boracic acid, and to circular discs of salicine (prepared by fusion), the difference being that the salts of quinidine have more intense depolarising powers than either of the other substances; the mode of preparation, however, excludes these from consideration. Quinine prepared in the same manner as quinidine has a very different mode of crystallisation; but it occasionally presents circular corneous plates, also exhibiting the black cross and white sectors, but not with one-tenth part of the brilliancy, which of course enables us readily to discriminate the two.

Fig. 188.—Urinary Salts, seen under Polarised Light.a, Uric acid;b, Oxalate of lime, octahedral crystals of;c, Oxalate of lime allowed to dry, forming a black cube;d, Oxalate of lime as it occasionally appears, termed the dumb-bell crystal.

Fig. 188.—Urinary Salts, seen under Polarised Light.

a, Uric acid;b, Oxalate of lime, octahedral crystals of;c, Oxalate of lime allowed to dry, forming a black cube;d, Oxalate of lime as it occasionally appears, termed the dumb-bell crystal.

Urinary salts are more readily seen under polarised light than by white light. Ice doubly refracts, while water singly refracts. Ice takes the rhomboidic form; and snow in its crystalline forms may be regarded as the skeleton crystals of this system (Fig. 189). A sheet of clear ice, of about one inch thick, and slowly formed in still weather, shows circular rings with a cross by polarised light.

Fig. 189.—Snow Crystals.

Fig. 190.—Potato Starch, under Polarised Light.

It is probable that the conditions of snow formation are more complex than might be imagined, familiar as we are with the conditions relating to the crystallisation of water on the earth’ssurface. A great variety of animal, vegetable, and other substances possess a doubly refracting or depolarising structure, as: a quill cut and laid out flat on glass; the cornea of a sheep’s eye; skin, hair, a thin section of a finger-nail; sections of bone, teeth, horn, silk, cotton, whalebone; stems of plants containing silica or flint; barley, wheat, &c. The larger-grained starches form splendid objects;tous-les-mois, the largest, may be taken as a type of all others. This presents a black cross, the arms of which meet at the hilum (Fig. 190). On rotating the analyser, the black cross disappears, and at 90° is replaced by a white cross; another, but much fainter, black cross is seen between the arms of the white cross, no colour being perceptible. But if a thin plate of selenite be interposed between the starch-grains and the polariser, a series of delicate colours appear, all of which change on revolving the analyser, becoming complementary at every quadrant of the circle. West and East India arrow-root, sago, tapioca, and many other starch-grains, present a similar appearance; but in proportion as the grains are smaller, so are their markings and colourings less distinct.

For the purpose of studying the various interesting phenomena of molecular rotation, a few necessary pieces of apparatus must be added to the microscope. First, an ordinary iron three-armed retort stand, to the lower arm of which must be attached either a polarising prism or a bundle of glass plates inclined at the polarising angle; in the upper an analysing prism. The fluid to be examined should be contained in a narrow glass tube about eight inches in height, and this must be attached to the middle arm. If the prisms be crossed before inserting a fluid possessing rotatory power, the light passing through the analyser will be coloured. If a solution of sugar be employed, and the light which passes through the second prism is seen to be red, but on rotating the analyser towards the right the colour changes to yellow, and passes through green to violet, it may be concluded that the rotation is right-handed. If, on the contrary, the analyser requires to be turned towards the left hand, we conclude that the polarisation is left-handed. These phenomena are wholly distinct from those accompanying the action of doubly refracting substances upon plane polarised light. It is not easy to explain in a limited space the course to be followed inascertaining the amount of rotation produced by different substances. Monochromatic light should be used. If we are about to examine a sugar solution with the prisms crossed, the index attached to the analyser must first be made to point to zero. The sugar is then introduced, when it will be necessary to rotate the analyser 23° to the right, in order that the light may be extinguished. This is the amount of rotation for that particular fluid at a given density and that height of column. As the arc varies with increase or decrease of density and height of the fluid, it is needful to reduce it to a unit of height and density. The following formula is that given by Biot:—P = quantity of matter in a unit of solution;d= sp. gr.;l= length of column;a= arc of rotation;m= molecular rotation.

Thenm=a/(l p d).

The application of the polarising apparatus to the microscope is of much value in determining minute structure. It may also be defined as an instrument of analysis; a test of difference in density between any two or more parts of the same substance. All structures, therefore, belonging either to the animal, vegetable, or mineral kingdom, in which the power of unequal or double refraction is suspected to be present, are those that should especially be re-investigated by polarised light. Some of the most delicate of the elementary tissues of animal structure, the ultimate fibrillæ of muscles, &c., are amongst the most interesting subjects that might be studied with advantage under this method of investigation. The chemist may perform the most dexterous analysis; the crystallographer may examine crystals by the nicest determination of their forms and cleavage; the anatomist or botanist may use the dissecting knife and microscope with the most exquisite skill; but there are still structures in the mineral, vegetable, and animal kingdoms which will defy all such modes of examination, and will yield only to the magical analysis of polarised light.

The inorganic kingdom will afford to the microscopist a never-ending number of objects of unsurpassed beauty and interest. The phenomena of crystallisation in its varied combinations can be madea useful and instructive occupation. Although ignorant of the means whereby the great majority of minerals and crystals have been formed in the vast laboratory of Nature, we can, nevertheless, imitate in a small degree Nature’s handiworks by crystallising out a large number of substances, and watch their numerous transformations in the smallest appreciable quantities, when aided by the microscope.

Among natural crystals we look for the material for the formation of our lenses, while the varieties of granites present us with the earliest crystallised condition of the earth’s crust as it cooled down, the structure of which is beautifully exhibited under polarised light. InPlate VIII. various crystalline and other bodies are displayed. In No. 158 is a section of new red sandstone; 159 of quartz; and 160 of granite. Special reference is made to others in the following list of salts and other substances which form a beautiful series of objects for study under polarised light:—

Alum.Asparagine.Aspartic Acid.Plate VIII. No. 168.Bitartrate of Ammonia.Boracic Acid.Borax. No. 164.Carbonate of Lime."Soda.Chlorate of Potash.Chloride of Barium."Cobalt."Copper and Ammonia."Sodium.Cholesterine.Chromate of Potash.Cinchonine.Cinchonidine.Citric Acid.Hippuric Acid.Iodide of Mercury."Potassium."Quinine.Iodo-disulphate of Quinine.Kreatine. No. 166.Murexide.Nitrate of Bismuth."Barytes."Brucine."Copper."Potash."Strontian."Uranium.Oxalate of Ammonia."Chromium."Chromium and Potash."Lime."Soda.Indurated Sandstone, Howth.Indurated Sandstone, Bromsgrove.Gibraltar Rock.Granite, various localities. No. 160.Hornblend Schist.Labrador Spar.Norway Rock.Quartz Rock, various. No. 159."in Bog Iron Ore.Quartzite, Mont Blanc.Sandstone. No. 158.Satin Spar.Selenites, various colours.Tin Ore, with Tourmalin.Oxalic Acid.Oxalurate of Ammonia.Permanganate of Potash.Phosphate of Lead and Soda.Platino-cyanide of Magnesia.Plumose Quinidine.Prussiate of Potash, red and yellow.Quinidine.Santonine.Salicine.Salignine. No. 162.Sulphate of Cadmium."Copper. No. 161."Copper and Potash."Iron. No. 163."Iron and Cobalt. No. 165."Magnesia."Nickel and Potash."Soda."Zinc.Sugar.Tartaric Acid.Thionurate of Ammonia.Triple Phosphate.Urate of Ammonia."Soda.Urea, and most urinary deposits.Uric Acid.

Agates, various.Asbestiform Serpentine.Avanturine.Carbonate of Lime.Carrara Marble.

Cat’s Tongue. No. 174.Grayling Scale. No. 176.Holothuria, Spicules of. Nos. 171-2.Prawn Shell. No. 175.

Cuticleof Leaf of Correa Cardinalis." "Deutzia scabra. No. 173." "Elæagnus." "Onosma taurica.Equisetum. No. 170.Fibro cells from orchid. No. 169."Oncidium bicallosum.Scalariform Vessels from Fern.Scyllium Caniculum. No. 177.Silicious Cuticles, various.Starches, various. No. 167.

The formation of artificial crystal may be readily effected, and the process watched, under the microscope, by simply placing a drop of saturated solution of any salt upon a previously warmed slip of glass.

Interesting results will be obtained by combining two or more chemical salts in the following manner. To a nearly saturated solution of the sulphate of copper and sulphate of magnesia add a drop on the glass-slide, and dry quickly. To effect this, heat the slide so as to fuse the salts in its water of crystallisation, and there remains an amorphous film on the hot glass. Put the slide aside and allow it to cool slowly; it will gradually absorb a certain amount of moisture from the air, and begin to throw out crystals. If now placed under the microscope, numerous points will be seen to start out here and there. The starting points may be produced at pleasure by touching the film with a fine needle point, so as to admit of a slight amount of moisture being absorbed by the mass of salt. Development is at once suspended by applying gentle heat; cover the specimen with balsam and thin glass. The balsam should completely cover the edges of the thin glass circle, otherwise moisture will probably insinuate itself, and destroy the form of the crystals.

Mr. Thomas succeeded in crystallising “the salts of the magnetic metals” at very high temperatures, with very curious results. InPlate VIII. are seen crystals of sulphate of iron and cobalt, No. 163;and of nickel and potash, No. 165, obtained in the following manner:—Add to a concentrated solution of iron a small quantity of sugar, to prevent oxidation. Put a drop of the solution on a glass slide, and drive out the water of crystallisation as quickly as possible by the aid of a spirit lamp; then with a Bunsen’s burner bring the plate to a high temperature. Immediately a remarkable change is seen to take place in the form of the crystal, and if properly managed the “foliation” represented in the plate will be fairly exhibited. The slide must not be allowed to cool down too rapidly or the crystals will probably absorb moisture from the atmosphere, and in so doing the crystals alter their forms. Immerse them in balsam, and cover in the usual way before quite cold.

Sublimation of Alkaloids.—The late Dr. Guy, F.R.S., directed the attention of microscopists to the fact that the crystalline shape of bodies belonging to the inorganic world might be of service in medical jurisprudence. Subsequently, Dr. A. Helwig, of Mayence, investigated this subject, and found the plan applicable not only to inorganic but also to organic substances, and especially to poisonous alkaloids. By using a white porcelain saucer Dr. Guy was able to watch the process of crystallisation more minutely, and to regulate it more exactly. He was, in fact, able to obtain characteristic crusts composed of crystals of strychnine weighing not more than1⁄3000th or1⁄5000th of a grain. Morphia affords equally characteristic results. For the examination of these, Dr. Guy recommended the use of a binocular microscope with an inch object-glass. But it is not to crystalline forms alone that one need trust; the whole behaviour of a substance as it melts and is converted into vapour is eminently characteristic, and when once deposited on the microscopical slide, under the object-glass, the application of re-agents may give still more satisfactory results. The re-agents, however, which are here to be applied are not of the kind ordinarily employed. Colour-tests under the microscope are, comparatively speaking, useless; those that give rise to peculiar crystalline forms are rather to be sought after. For instance, the crystals produced by the action of carbozotic acid on morphia are by themselves almost perfectly characteristic. These experiments should not, however, be undertaken for medico-legal purposes by one unskilled in their conduct, for the effects of the reagents themselves might be mistaken by the uninitiated for theresult of their action on the substance under examination. For the special method of procedure, see Dr. W. Guy, “On the Sublimation of the Alkaloids.”34

Spectrum analysis has, from its first introduction by Kirschoff in 1859, maintained its fascination over men of science throughout the civilised world. Microscopists, astronomers, and chemists have assigned to the spectroscope a highly important position amongst scientific instruments of research. At quite an early period of its history it appeared to ourselves to promise an extension of the work of the microscope in pathology and microscopy, and second only to that of astronomy and chemistry. The chief hindrances to the use of the spectroscope were, in the early days, of a twofold nature; a widespread, but quite erroneous view of the serious difficulties of employing the instrument, and the want of a first aid to its use.

So valuable a means of research has this process of analysis proved to be, that the discoveries made by the spectroscope appear marvellous. The spectroscope was first made known as a refined instrument for the analysis of light by two Germans, a physicist and a chemist, Kirschoff and Bunsen. In 1860, the latter succeeded in detecting and separating two new alkaline bodies from all other bodies from the waters obtained from the Durkeim springs, less than 0·0002 part of a milligramme of which can be detected by spectrum analysis. It is to the labours of Huggins, Norman Lockyer and others that we are indebted for the wonderful discoveries made in astronomy; and chiefly so to Brewster, Herschel, and Talbot, for showing that certain metals give off light of a high degree of refrangibility; that distinct bands are situated at a distance beyond the last visible violet ray ten times as great as the length of the whole visible spectrum from red to violet.

With regard to the discoveries made in connection with physiological research, we are indebted to F. Hoppe, who in 1862 first described the absorption bands of human blood. His results were confirmed by the investigations of Professor Sir George Gabriel Stokes, who,by adding certain reducing agents to the blood, found that he could change scarlet blood into purple—“purple cruorine”—and in this way the place occupied by the absorption band in the spectrum could be made to change. He reduced the hæmoglobin by robbing the blood of its oxygen. Thus, by Stokes’ and other methods, we have since arrived at extremely valuable results, and the explanation of the difference in colour between arterial and venous blood; and it has also enabled us to show wherein the breathing power of the red corpuscles resides, and further explains phenomena which before his investigations were inexplicable.

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