﻿M. L. Soret on the Colour of the Lake of Geneva, 347 



If the telescope, always in the same vertical plane passing 

 through the sun, be inclined in any other direction, then the po^ 

 larization is the less marked the more the direction of the tele- 

 scope differs from the perpendicular to I R. If the direction of 

 the telescope be vertical, so that one is looking downwards, no 

 trace of polarization appears. 



If the telescope be placed in a vertical plane perpendicular to 

 the former one (that is, at 90° to the sun), the polarization is 

 the more distinct the more the direction of the telescope becomes 

 horizontal, and the plane of polarization passes through the direc- 

 tion of the telescope and the sun. 



I have not been able as yet to prove that the maximum of po- 

 larization corresponds exactly to the position of the telescope 

 where its axis is at a right angle to the direction of the solar rays. 



It cannot escape your attention that the phenomena are more 

 complicated in the case of water than in the case of the firmament. 



In the first place, it is evident that, if the surface of the water 

 is agitated, the solar rays cannot remain parallel after the refrac- 

 tion. The phenomenon, therefore, is the less distinct the more 

 agitated the water is. This is exactly what I observed. When 

 I tried my apparatus for the first time, the water was agitated, 

 and I remarked no appreciable polarization — although this may 

 be attributed as well to the fact that, for some days before, we 

 had strong northerly winds and the water was not perfectly 

 transparent. On two other days, when the water was only 

 slightly agitated, an appreciable polarization could be observed. 

 On a very fine and calm day at last, the polarization was as 

 marked as that of the sky, which, however, was not very blue at 

 the time. 



In the second place, the solar rays entering the water are 

 already partially polarized by refraction ; but when the telescope 

 is placed in the vertical plane passing through the sun (that is, 

 in the position most favourable for observation), it is easy to see 

 that the rays already polarized must be extinguished (exactly as 

 in the experiment where you produce a blue cloud with a pencil 

 of light already polarized) . 



Finally, the direct solar light is not the only light that enters 

 the water. There are scattered rays of diffused light, which, 

 impinging upon the water from all directions, produce after 

 refraction a blue non-polarized light, or, properly speaking, an 

 infinite number of rays polarized with different planes of polari- 

 zation. I have satisfied myself that, when the sky is covered, 

 no appreciable polarization is observed. 



As I know of no prior description of this phenomenon, and as 

 peculiar circumstances prevent for some time my pursuing this 

 investigation, I communicate to you these results, although I 



