In the first of these cases, the retardation is due to molecular heterogeneity; in the second, it depends upon molecular motions produced by the magnet. The effect is greater in each case the more refrangible the rays; and therefore, when the light which has passed through the medium is examined with an analyzer, the successive colors of the spectrum are cut off each at a different angle, and the observed tint is that com pounded of those which remain. The saccharometer (q.v.), for the determination of sugar in a liquid, is an application of the first case; the second has not as yet been Implied to any practical purpose, but it has given most valuable information as to the ultimate nature of magnetisin.
\Vhen polarized light passes through a slice of any uniaxal double-refracting crystal, nearly in the direction of its 11:d6, it is obvious that the difference of retardation of the two rays into which it is divided will depend only upon (1) their refrangibility and (2) their inclination to the axis of the crystal. Hence, if we suppose the light to be homogeneous, the effects of interference, and subsequent application of the analyzer, must i.e to pro duce appearances of bright and dark spaces, symmetrically disposed round the axis; that is, a series of concentric circular rings. The superposition of the separate sets of rings, for each color of the spectrum, produces the appearance actually observed; a series of colored rings, like those known as Newton's rings, due to interference (q.v.). Besides these, however, there is a dark or bright cross, consisting of two black or white bands, intersecting each other in the common center of the rings. The dark bands are due to the absolute stoppage by polarizes or analyzer, when placed in positions 00° from symmetry. of all light whose vibra tions are executed in the principal planes of the polar izes and analyzer. A simila• explanation applies to any other case. The system of colored rings thus produced is one of the most splendid results of the optical combi nations yet produced; and may be seen by any one by the help of such simple apparatus as two fragments of window-glass and a piece of clear ice from the surface of a pone'. In undisturbed freezing, the axis of the ice crystal is perpendicular to the surface of the water, and the cake of ice is therefore, as it were, cut for our purpose. If light be reflected at an angle of about 54* from the first piece of glass. pass perrendieniarly through the ice, and he again reflected (at 54) from the second piece of glass, the phenomena above described will be at once seen, the appear ances varying with tie relative position of the planes in which the reflections take place from the pieces of glass. If these planes be at right angles to each other, we have the
black cross as in figure 2; if parallel, a white cross.
If, instead of a uniaxal crystal, it biaxal crystal, such as niter or arragonite, be employed, the system of colored rings and dark brushes is more complex; symmetry now requiring their arraegement about the two optic axes. The general the rings and brushes depends now, not only on the relative position of the polarizes and analyzer, lint also on the position of the crystal (which is no longer symmetrical about an axis) with reference to these planes.
By employing circularly or elliptically polarized light, the appearances may be still further varied, hut we cannot enter into details.
Every doubly refl.:10ing body produces a change upon polarized light which passes through it. Hence the application of the polarizes and analyzer (usually glans mirrors, Dr NiC0I'S prisms) to the microscope is often of very great use in detecting crystalline, and other structural peculiarities. Solid bodies, such ns glass, which are singly refractive, become doubly refractive when strained either by external forces or by nnequal healing. A 1xvnionent slate of strain is produced in glass when it is cooled All these phenomena are hewn i fully exhibited by polarized light. Again, the application of polar ized light is sometimes of great importance in qualitative analysis, where only an exceed ingly small quantity of a substance is procurable for examination, by enabling the chemist to determine whether a ininute crystal is doubly refractive or not.
A practical application of It polarizing prism may be mentioned. In sahnon spearing it is often execedinzly difficult to see the fish at the bottom of the stream, on account of the glare of light reflected from the surface. But as this light is always partially, some times ?vholly polarized. a great part of it may be arrested by the analyzer held in a proper azinnith; while the light escaping from the water will suffer little loss.
The light of the sky, being mainly reflected light, is of course partially polarized. The investigation of this subject has been most ably conducted by Brewster (Trans. B. S. E.,