226 M. L. Soret on Illumination and Polarization 



polarized in a vertical plane — that is to say, when the vibrations 

 of the incident light are horizontal. 



For the eye placed in the horizontal plane which passes through 

 the centre of the sphere, the first vertical components will de- 

 stroy each other by interference. Let us, in fact, take two sym- 

 metrical points, n and n' ; the incident horizontal vibrations will 

 give rise to vertical reflected vibrations (or at least to vertical 

 components); but, the inclination of the reflecting surface in 

 reference to the horizontal vibrations being in opposite directions 

 for the points wand n\ the reflected motion must also be in the 

 opposite direction. 



The second components, which are horizontal and parallel to 

 the pencil, may reach the eye ; there is no reason why they 

 should interfere. The intensity of the light due to these com- 

 ponents must be very feeble, as we shall subsequently see. 



Nor will the third, horizontal components perpendicular to 

 the pencil neutralize each other. 



Thus the eye placed in the horizontal plane passing through 

 the centre of the sphere will receive light from the second and 

 third components. If the visual angle is 90°, the third com- 

 ponents will produce no effect upon the eye, since they act 

 longitudinally. Thus the eye in this position will only receive 

 the impression of the horizontal components parallel to the 

 pencil, which, as we have already said, are very feeble, and often 

 quite imperceptible. 



We readily deduce from this the explanation of the experi- 

 ment which was suggested to you by Mr. Stokes*. 



If the visual angle is greater or less than 90°, the eye will 

 receive the combined effect of the second and third components ; 

 but the direction of the resulting vibrations will be always 

 parallel to the horizontal plane passing through the incident 

 pencil. This result is also quite in accordance with experiment. 



VI. If the illuminating pencil consists of natural light, we 

 shall easily find what takes place by regarding it as composed 

 of two luminous pencils polarized at right angles, one in the 

 plane of vision (which I suppose to be always horizontal),, the 

 other in a perpendicular plane. We have the maximum polar- 

 ization when the angle of vision is 90° ; for in this position the 

 eye receives vertical components in large quantities, while 

 from the horizontal components it only receives the impression 

 of the second components of the reflected vibrations proceeding 

 from the pencil polarized perpendicularly to the plane of vision. 



In a private note Professor Stokes suggested to me the employment 

 of polarized light to illuminate the actinic cloud, and I feel pleasure in 

 giving him the credit of the suggestion. Had the suggestion not come 

 to me, however, the result would have been substantiallv the same.— J. T. 



