POLARISATION OF LIGHT. CHAPTER I METHODS OF POLARISATION. LIGHT 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. The origin and meaning of the term polarisation will be better understood when some of the phenomena have been witnessed or described than beforehand, and I therefore postpone, for the present, an explanation of it. The subject of polarisation may be approached by either of two roads, the experimental or the theoretical. The theoretical method, which proceeds upon the principles of the Wave Theory of Light, is remarkably complete and explicit; so much so that it not only connects together many very diversified phenomena, B but even, in some cases, has suggested actual prediction. But inasmuch as the theory without experimental facts would be little better than a study of harmony without practical music, it will be best to begin with experiment. It was stated above that certain instrumental means were requisite for detecting polarisation. 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." The four principal processes by means of which a ray of light may be polarised are, reflexion, ordinary refraction, double refraction, and scattering by small particles. These methods will be considered in order; but before doing so, it will be convenient to describe the phenomena of polarisation as exhibited by some instrument tolerably simple in its action, and of easy manipulation. For such a purpose a plate of crystal called tourmalin will perhaps serve better than any other to begin with. Tourmalin is a crystal of which there are several varieties, differing only in colour. Very dark specimens generally answer the purpose well, excepting that it is difficult to cut them thin enough to transmit much light. Red, brown, or green specimens are usually employed; the blue are, for the most part, optically unsuitable. Some white, or nearly white, specimens are very good, and may be cut into thicker plates without loss of light. If we take a plate of tourmalin cut parallel to a particular direction within the crystal called the optic axis (the nature and properties of which will be more particularly explained hereafter), and interpose it in the path of a beam of light at right angles to the direction of the beam, the only effect perceptible to the unassisted eye will be a slight colouring of the light after transmission, in consequence of the natural tint of the particular piece of crystal. But if we examine the transmitted beam by a second similar plate of tourmalin, placed parallel to the former, the following effects will be observed. When the two plates are similarly placed, i.e. as if they formed one and the same block of crystal, or, as it is technically expressed, with their optic axes parallel, we shall perceive only, as before, the colouring of the light due to the tints of the two plates. But if either of the plates be then turned round in its own plane, so as always to remain perpendicular to the beam, the light will be observed to fade gradually, until, when the moving plate has been turned through a right angle, the light becomes completely extinguished. If the turning be continued beyond the right angle, the light will begin to revive; and when a second right angle has been completed, the light will be as bright as at the outset. In Figs. I and 2, a, b, c, d, e, f, g, h, represent the two plates; in Fig. I the two plates are supposed to be in the first position; in Fig. 2 the plate e, f, g, h, has been turned through a right angle. Of the parts which overlap, the shading in Fig. I represents the deepened colour due to the double thickness of the crystal; in Fig. 2 it indicates the complete extinction of the light. The same alternation of brightness and extinction will continue for every right angle through which the moving plate is turned. Now it is to be observed that this alternation depends only upon the angle through which one of the crystals has been turned, or, as it is usually stated, upon the relative angular position of the two crystals. Either of them may be turned, and in either direction, and the same sequence of effect will always be produced. But if the pair of plates be turned round bodily together, no change in the brightness of the light will be made. It follows, therefore, that a ray of ordinary light possesses the same properties all round; or, as it may be described in more technical language, a ray of ordinary light is symmetrical in respect of its properties about its own direction. On the other hand, a ray of light, after traversing a plate of tourmalin, has properties similar, it is true, on sides, or in directions, diametrically opposite to one another, but dissimilar on intermediate sides or directions. The properties. in question vary, in fact, from one angular direction. to another, and pass through all their phases, or an entire period, in every angle of 180°. This directional character of the properties of the ray, on account of its analogy (rather loose, perhaps) to the directional character of a magnet or an electric current, suggested the idea of polarity; and hence the condition in which the ray was found to be was called polarisation. Having so far anticipated the regular order of things on the experimental side of the subject, it will perhaps be worth while to make a similar anticipation on the side of theory. 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, and none simpler have been suggested, 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; and |