FRINGES IN UNANNEALED GLASS.

371

of reflection and polarization may coincide (524, fig. 1), the colours, which almost entirely vanish at 45°, will undergo a remarkable change; the figure shown at в will appear, all the colours of which are complementary to those of A, and the black cross will be replaced by white spaces.

545. If the plate of unannealed glass be square, and about one third as broad as it is long, the elegant figure shown at A will be visible when the analyzing plate is set at 90°, so that

the plane of reflection may be perpendicular to the plane of polarization. The complementary figure B, replacing it when the analyzing plate is placed at 0 or 180°, so that the planes of reflection and polarization correspond.

The dark lines forming the black cross, seen when these plates or laminae of crystals (541, 543) are submitted to polarized light, must be considered as pointing out the position of the points where the polarized ray passes through unchanged, and are hence conveniently called lines of no polarization. If the analyzing plate be fixed, and the unannealed glass be slowly turned round, the black cross will begin to open, and its arms to separate in elegant curves,

until its resultant axes (510) are inclined 45° to the planes of polarization and reflection (524), when a beautiful symmetrical figure will be visible, as at c in the last figure. On continuing to turn the plate of glass, the dark cross gradually re-appears, and attains its greatest intensity when one of its arms corresponds to the plane of polarization, and the other to that of reflection.

546. When the crystal submitted to polarized light is very small, both it and the eye should be placed as near as possible to the analyzing plate. Or the specimen may be placed on the stage of the polariscope (523), and a convex lens of about one or two inches focus held above it; a magnified image will thus become visible in the analyzing plate.

Instead of viewing the reflected image in the analyzing plate, the crystal through which the polarized beam is passing may be examined by looking at it through a thin plate of agate, or tourmaline (521), and thus the complementary colours will become developed.

Another, and very interesting mode of analyzing the transmitted light, is to replace the analyzing glass plate of the polariscope, by several thin laminae of mica, or talc, placed closely together. When fixed at the polarizing angle, the usual image with the polarized tints may be seen by reflection, as in the glass plate: and on looking vertically downwards through the inclined laminæ of mica, a second image, complementary to the first, will become visible without moving the plate. If a thin piece of mica be exposed to a red heat, so as to split it into numberless laminæ, it will, as Professor Forbes has shown, render the complementary tints visible in a very beautiful manner. The eyepiece before described, consisting of six or eight plates of glass fixed diagonally in a tube (527), may be very conveniently substituted for the analyzing plates of the polariscope, in developing the tints of doubly refracting crystals. On viewing, for example, un annealed glass, when traversed by a polarized ray, by means of this eye-piece, the

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373

beautiful tints traversed by the black cross, will be seen when the plane of the glass plates in the tube corresponds to that of the polarizing plate; the complementary colours becoming visible on turning the tube round 90°, so that the plane of the inclined glass plates may be at right angles to that of the polarizing plate.

547. When a mass of animal jelly is placed on the stage of the polariscope, no colours are visible in the analyzing plate, so long as the jelly is not submitted to pressure; but as soon as it is compressed with sufficient force, it assumes a doubly refracting structure, and a series of tints traversed by a black cross becomes visible, providing the analyzing plate be so placed that the planes of reflection and polarization are at right angles.

Jelly, solutions of gum, and albuminous fluids, allowed to evaporate spontaneously, so as to leave an indurated mass, also exhibit the four coloured sectors, traversed by a black cross. A slip of glass, previously without action on polarized light, develops a series of tints, by bending it or submitting it to pressure.

548. No series of objects exhibits the tints of polarized light more beautifully than the crystalline lenses of animals, especially of fishes; to examine these they should, to prevent their bringing the incident rays to a focus, be immersed in a glass vessel containing oil, or some fluid possessing nearly the same refractive power as the lens. The crystalline lens of the cod fish exhibits twelve beautiful coloured sectors, separated by two dark concentric circles of no polarization, (545), and traversed by a black cross.

Fragments of ordinary quills, and other indurated animal structures, also exhibit these tints, when submitted to the action of polarized light, in an extremely beautiful

manner.

549. When two systems of undulations of equal amplitude, and polarized in planes at right angles to each other, differ in their paths by a quarter of an undulation, the com

pound movement thus generated in each molecule of ether, will not be rectilinear, as in the variety of polarized light we have just examined, but circular. When the set of undulations which is in advance of the other by the fourth of an entire wave, has its plane of polarization to the right of that of the latter series, the motion will be propagated from right to left in a spiral direction, and from left to right when the systems of waves are ranged in the opposite direction. Let a thin plate of regularly crystallized quartz be cut in a direction perpendicular to its axis, and placed on the stage of the polariscope; on looking into the analyzing plate, no black cross will be visible as in calcareous spar (541). But a few rings will be seen at the circumference of the crystal, the centre being filled up by an uniform tint, providing the plate be of the same thickness throughout, otherwise it will vary, as the intensity of colour depends on the thickness of the plate. If the central colour be red, slowly revolve the analyzing plate, and the tint will change to orange, yellow, green, and ultimately to violet; as though the analyzing plate had during its rotation acquired the power of reflecting these different colours.

In some specimens of quartz, and other crystals possessing this power of circular polarization, the colours change from red to violet, when the analyzing plate is turned from right to left, and in others when it is moved from left to right. Hence these crystals are termed right-handed, or left-handed, according as they possess the property of causing the planes of polarization to revolve spirally in a direction from right to left, or left to right.

A plate of left-handed quartz 0·3 inch thick, when placed on the stage of the polariscope, so that a polarized ray may pass through it, appears of a fine blue, when viewed through a plate of tourmaline, or bundles of mica or glass plates (546), held in such manner as to prevent the ray from being refracted through them before being transmitted through the crystal. On turning the quartz round on its axis, no change

CIRCULAR POLARIZATION IN QUARTZ.

375

of colour ensues; but on moving the eye-piece of tourmaline, glass plates (546), &c., the following changes of colour are observed at different azimuths:

The phenomena thus observed, are the same as would necessarily occur, if the polarized light had been, by passing through the quartz, resolved into a series of homogeneous rays, and become disposed in different planes radiating from the centre of a circle, as shown in the marginal figure representing Newton's chromatic circle in the circumference of which the colours of the spectrum (467) are arranged. The

VIOLET

INDICO

BLUE

RED

YELLO

thicker the plate of quartz employed,

the greater is the arc required to effect the conversion of the image into one of a different tint; so that, although in the above experiment a rotation of the analyzing eye-piece through an arc of

180° was sufficient to develop images. possessing every colour of the spectrum (467), yet, on increasing the thickness of the plate, a much larger arc is required to produce the same effect.

To render this more intelligible, place a plate of quartz, 0.04 inches thick, on the stage of the polariscope, and allow a ray of homogeneous light (467) to fall upon the polarizing plate. If this light be red, then the space in the centre of the rings visible in the quartz will be the brightest when the index is at 90°, or when the planes of reflection and polarization are at right angles to each other. Slowly revolve the analyzing