... 202 220 237 253 POLARISATION OF OF LIGHT. A LECTURE, Delivered in the Hulme Town Hall, Manchester, on Tuesday, October 28th, 1873, By WM. SPOTTISWOODE, Esq., F.R.S. IGHT is said to be polarised when it presents certain peculiarities, hereafter to be described, which it is not generally found to possess. These peculiarities, although very varied in their manifestations, have one feature in common, viz., that they cannot be detected by the unassisted eye, consequently special instrumental means are required for their investigation. Now there are various processes, some occurring in the ordinary course of natural phenomena, others due to instrumental appliances, whereby a ray of light may be brought into the condition in question, or "polarised;" and it is a fact, both curious in itself and important in its applications, that any one of these processes (not necessarily the same as that used for polarising) may be used also as a means of examining whether the ray be in that condition or not. This latter process is called analysation." When two instruments, whether of the same or of different kinds, are used, they are called respectively the "polariser" and the "analyser," and the two together are included under the general name of "polariscope." 66 It is considered as established that light is due to the vibrations of an elastic medium, which, in the absence of any better name, is called ether. The ether is understood to pervade all space and all matter, although its motions are affected in different ways by the molecules of the various media which it permeates. The vibrations producing the sensation of light take place in planes perpendicular to the direction of the ray. The paths or orbits of the various vibrating ethereal molecules may be of any form consistent with the mechanical constitution of the ether, but on the suppositions usually made the only forms possible are the straight line, the circle, and the ellipse. But in ordinary light the orbits at different points of the ray are not all similarly situated. This being assumed, the process of polarisation is understood to be the bringing of all the orbits throughout the entire ray into similar positions. And in the case where the orbits are all straight lines, they all lie in one and the same plane. For this reason the polarisation produced by most processes is called rectilinear, or, more commonly, plane polarisation. The most simple, or ready to hand, method of polarising light is by reflection. Nearly all polished substances, except metals, have the property of producing this result. If light be reflected from the surface of a plate of glass, for instance, and then be received so as to be reflected from a second plate placed parallel to the first, no particular effect will be seen. But if the second plate be then turned round in such a way as always to preserve the same inclination to the ray reflected from the first, then the light reflected from the second plate will be found gradually to fade until the angle of turning has amounted to a right angle. If the turning be continued still further the light will gradually become brighter again, until when the angle of turning has reached a second right angle (or 180° in all) the light will have resumed its original brightness. This gradual fading and revival of the light, when the second plate is turned round in the manner above described, takes place in a greater or less degree when the rays fall upon the first plate at almost any angle. But, if that angle be varied, it will be found that the difference in the intensity of the light produced by turning the second plate is much increased between the ray and the plate until the angle reaches 35°, or thereabouts. When the ray falls at this angle the light is entirely extinguished by turning the second plate through a right angle. The light, therefore, in being reflected at this particular angle, has acquired the property of being extinguished by a second reflection, and this property forms one of the main features of polarisation. At first sight this may appear more curious than important, but the consequences of it will be more fully seen hereafter. The effects here described may be produced by other polished substances; but amongst them there is, perhans, none more |