﻿388 Royal Society : — Prof. Tyndall on the Blue Colour 



blue, the whiter light of the cloud being extinguished*. When the 

 cloud- texture has become sufficiently coarse to approximate to that 

 of ordinary clouds, the rotation of the Nicol ceases to have any sen- 

 sible effect on the quality of the light discharged normally; 



The perfection of the polarization in a direction perpendicular to 

 the illuminating beam is also illustrated by the following experiment. 

 A Nicol's prism large enough to embrace the entire beam of the 

 electric lamp was placed between the lamp and the experimental 

 tube. A few bubbles of air carried through the liquid nitrite of butyl 

 were introduced into the tube, and they were followed by about 3 

 inches (measured by the mercurial gauge) of air which had been 

 passed through aqueous hydrochloric acid. Sending the polarized 

 beam through the tube, I placed myself in front of it, my eye being 

 on a level with its axis, my assistant, Mr. Cottrell, occupying a 

 similar position behind the tube. The short diagonal of the large 

 Nicol was in the first instance vertical, the plane of vibration of the 

 emergent beam being therefore also vertical. As the light continued 

 to act, a superb blue cloud visible to both my assistant and myself was 

 slowly formed. But this cloud, so deep and rich when looked at 

 from the positions mentioned, utterly disappeared when looked at 

 vertically doivnwards, or vertically upwards. Reflection from the 

 cloud was not possible in these directions. When the large Nicol 

 was slowly turned round its axis, the eye of the observer being on the 

 level of the beam, and the line of vision perpendicular to it, entire ex- 

 tinction of the light emitted horizontally occurred when the longer 

 diagonal of the large Nicol was vertical. But now a vivid blue cloud 

 was seen when looked at downwards or upwards. This truly fine 

 experiment was first definitely suggested by a remark addressed to 

 me in a letter by Prof. Stokes. 



Now, as regards the polarization of skylight, the greatest stum- 

 blingblock has hitherto been that, in accordance with the law of 

 Brewster (which makes the index of refraction the tangent of the 

 polarizing-angle), the reflection which produces perfect polarization 

 would require to be made in air upon air ; and indeed this led 

 many of our most eminent men, Brewster himself among the num- 

 ber, to entertain the idea of molecular reflection. I have, however, 

 operated upon substances of widely different refractive indices, and 

 therefore of very different polarizing-angles as ordinarily defined, but 

 the polarization of the beam by the incipient cloud has thus far 

 proved itself to be absolutely independent of the polarizing-angle. 

 The law of Brewster does not apply to matter in this condition ; and it 

 rests with the undulatory theory to explain why. Whenever the pre- 

 cipitated particles are sufficiently fine, no matter what the substance 

 forming the particles may be, the direction of maximum polarization 

 is at right angles to the illuminating beam, the polarizing angle for 

 matter in this condition being invariably 45°. This I consider to 

 be a point of capital importance with reference to the present questionf. 

 * This seems to prove that particles too large to polarize the blue, polarize 

 perfectly light of lower refrangibility. 



f The difficulty referred to above is thus expressed by Sir John Herschel : — 

 " The cause of the polarization is evidently a reflection of the sun's light upon 

 something. The question is, On what ? Were the angle of maximum polarization 



