﻿its Polarization and Colour. 109 



verse to the ray from another. We have now got what we want. 

 Suppose, for distinctness of statement, that the primary ray is 

 vertical, and that the plane of vibration is that of the meridian. 

 The intensity of the light scattered by a small particle is con- 

 stant, and a maximum for rays which lie in the vertical plane 

 running east and west, while there is no scattered ray along the 

 north and south line. If the primary ray is unpolarized, the 

 light scattered north and south is entirely due to that component 

 which vibrates east and west, and is therefore perfectly polarized, 

 the direction of its vibration being also east and west. Similarly 

 any other ray scattered horizontally is perfectly polarized, and 

 the vibration is performed in the horizontal plane. In other di- 

 rections the polarization becomes less and less complete as we 

 approach the vertical, and in the vertical direction itself altogether 

 disappears. 



So far, then, as disturbance by very small particles is con- 

 cerned, theory appears to be in complete accordance with the 

 experiments of Tyndall and others. At the same time, if the 

 above reasoning be valid, the question as to the direction of the 

 vibrations in polarized light is decided in accordance with the 

 view of Fresnel. Indeed the observation on the plane of polari- 

 zation of the scattered light is virtually only another form Of 

 Professor Stokes's original test with the diffraction-grating. In 

 its present shape, however, it is free from certain difficulties both 

 of theory and experiment, which have led different physicists who 

 have used the other method to contradictory conclusions. I con- 

 fess I cannot see any room for doubt as to the result it leads to*. 



The argument used is apparently open to a serious objection, 

 which I ought to notice. It seems to prove too much. For if 

 one disturbing particle is unable to send out a scattered ray in 

 the direction of original vibration, it would appear that no com- 

 bination of them (such as a small body may be supposed to be) 

 could do so, at least at such a distance that the body subtends 

 only a small solid angle. Now we know that when light vibra- 

 ting in the plane of incidence falls on a reflecting surface at an 

 angle of 45°, light is sent out according to the law of ordinary 



* I only mean that if light, as is generally supposed, consists of trans- 

 versal vibrations similar to those which take place in an elastic solid, the 

 vibration must be normal to the plane of polarization. There is unques- 

 tionably a formal analogy between the two sets of phenomena extending 

 over a very wide range ; but it is another thing to assert that the vibrations 

 of light are really and truly to-and-fro motions of a medium having mecha- 

 nical properties (with reference to small vibrations) like those of ordinary 

 solids. The fact that the theory of elastic solids led Green to Fresnel's 

 formulae for the reflection and refraction of polarized light seems amply 

 sufficient to warrant its employment here, while the question whether the 

 analogy is more than formal is still left open. 



