Rotation of Plane of Polarisation.
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 called rotation 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 be right-handed. In the case of what is called left-handed quartz, 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.
Quartz belongs 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 have two axes of 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 in [Fig. 185]. But if the tourmaline be again 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 of nitre be 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 in [Plate 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 strong solution of Cinchonidine add a drop or two of Herapath’s test-fluid.[33] A 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 indicate yellow, the straight lines red, the horizontal lines blue, 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 crystals at 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], a and b); 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 in [Fig. 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 alter on 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.
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’s surface. 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.