128 



NATURE 



[Dec. 1 8, 1873 



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 com- 

 plete 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 diametrically opposite to one another, but 

 dissimilar on intermediate sides or directions ; the proper- 

 ties in question vary in fact from one angular direction to 

 another, and pass through their phases or an entire period 

 in every angle of 18 degrees. 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 

 Jound 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 per- 

 pendicular 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 although there is 

 reason to believe that in general the orbits of a consider- 

 able number of consecutive molecules may be similarly 

 situated, yet in a finite portion of the ray there are a suf- 

 ficient number of variations of situation to prevent any 

 preponderance of average direction. 



This being assumed, the process of polarisation is un- 

 derstood to he the bringing of all the orbits through, 

 out the entire ray into similar positions. And in the 

 case of the tourmalin plate the orbits are all reduced 

 to straight lines, which consequently lie in one and the 

 same plane. For this reason the polarisation produced 

 by tourmalin, as well as by most other crystals, is called 

 rectilinear, or more commonly, plane polarisation. This 

 property of tourmalin may also be expressed by saying 

 that it permits only rectilinear vibrations parallel to a 

 particular direction determined by its own internal struc- 

 ture to traverse it. 



Adopting this view of polarisation as affected by a 

 plate of tourmalin, it would be interesting to ascertain the 

 exact direction of the vibrations. And a simple experi- 

 ment will go far to satisfy us on that point. The argument, 

 as now stated at least, is perhaps based upon general con- 

 siderations rather than upon strict mechanical proof; but 

 the experimental evidence is so strong that it should not 

 be denied a place here. Supposefor a moment that thetour- 

 malin be so placed that the direction of vibration lies either 

 in or perpendicular to the plane of incidence (that is, the 

 plane containing the incident ray, and a perpendicular to 

 the surface on which it falls at the point of incidence) ; 

 then it is natural to expect that vibrations executed in the 

 plane of incidence will be far more affected by a change 

 in the angle of incidence than those perpendicular to that 

 plane. In fact the angle between the direction of the 

 vibrations and the surface upon which they impinge, will 

 in the first case vary with the angle of incidence ; but in 

 the second case it will remain unchanged. 



In Figs. 3 and 4, n, o represents the ray of light ; 

 the arrow the direction of vibration, a, b, c, d, a', b', 

 c', d', the plate in two positions, turned in the first in- 

 stance about the direction of vibration, in the second 

 about a line perpendicular to it. 



Dismissing, then, the former supposition, and supposing 

 that nothing whatever is known about the direction of 

 vibration ; then, if all possible directions be taken in suc- 

 cession as pivots about which to tilt or turn the second 

 tourmalin, it will be found that for one direction the in- 

 tensity of the light diminishes more rapidly with an in- 

 crease of tilting (or, what is the same thing, with an 

 increase of the angle of incidence) than for any other. And 

 further, that for a direction at right angles to the first, 

 the intensity of light diminishes less than for any other ; 

 while for intermediate directions the diminution of inten- 

 sity is intciTnediate to those above-mentioned. In ac- 

 cordance, therefore, with what was said before, we may 

 conclude that the vibrations are parallel to the line or pivot 

 about which the plate was turned when the diminution of 

 light was least. 



Secondly, polarisation may be effected by reflexion. 

 If light reflected from the surface of almost any, except 

 metallic, bodies be examined with a plate of tourmalin, 

 it will in general be found to show traces of polarisation • 

 that is to say, if the plate be caused to revolve in 

 its own plane, and the reflected rajs be viewed through 

 it, then in certain positions of the plate, the reflected 

 hght will appear less bright than in others. If the 

 angle at which the original rays fall upon the reflect- 

 ing surface be varied, it will be found that the amount 

 of alteration in brightness of the light seen through the 

 revolving tourmalin (or analyser) will also vary. This 

 fact may also be expressed thus : in polarisation by re- 

 flexion, the degree of polarisation, or the amount of 

 polarised light in the reflected ra>s, varies with the angle 

 of incidence on the reflecting surface. But at a particular 

 angle, called on that account the polarising angle, the 

 polarisation will be a maximum. This angle (usually 

 measured between the incident ray and the perpen- 

 dicular to the reflecting surface) is not the same for 

 all substances ; in fact it varies with their refrac- 

 tive power according to a peculiar law, which, when 

 stated in the technical language of science, may be 

 thus enunciated : the tangent of the polarising angle 

 is equal to the refractive index. Simple geometrical 

 considerations, combined with the usual expressions for 

 the laws of reflexion and refraction, will show that this 

 relation between the polarising angle and the refractive 

 index may be also expressed in the following way ; If 

 light be incident at the polarising angle, the reflected and 

 refracted rays will be at right angles to one another. 



In Fig. 5, s, i represents the incident, i, f the 

 reflected, and i, r the refracted ray. Then s, i will 

 be incident at the polarising angle when the angle s, i, r 

 is a right angle. 



