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



beauty of its colour, is very favourable to investigations of this 

 kind. 



- Tor this purpose I had a very simple apparatus constructed. 

 It is a kind of telescope; a flat plate of glass with parallel sur* 

 faces, fitted hermetically at its one end, serves as object-glass. 

 The instrument, therefore, can be immersed in water without the 

 latter being able to enter it. The eyepiece consists of a Nicol's 

 prism. It is easy to understand that, by immersing the tele- 

 scope in the water, the eye receives the blue rays of light emana- 

 ting from the water, and that this light can be analyzed by 

 turning the Nicol. 



By proceeding in this manner, I found that the water of our 

 lake really exhibits phenomena of polarization comparable to 

 those observed on the light of the sky; ouly their observation is 

 more difficult, and up to this time I have not been able to study 

 them as well as I could have wished. 



Supposing the surface of the water N N perfectly plane, which 



is the case in time of perfect calm, a beam of solar light incident 

 with the direction S I will be deviated to I R after the refraction. 

 Now from a boat the telescope can always be placed in the ver- 

 tical plane passing through the sun. If the telescope be inclined 

 in such a manner that its axis becomes perpendicular to I R, then 

 the light received by the eye is emitted perpendicularly to the di- 

 rection of the solar rays in water. This arrangement is analo- 

 gous to that where the maximum of polarization of the light of 

 the sky appears — that is, when one is looking at a right angle 

 from the sun. 



In this manner I made a series of observations on the Lake 

 of Geneva in a place where the water was sufficiently deep not 

 to allow the ground to be seen ; and I was able to perceive a 

 marked polarization. The plane of polarization was coincident 

 with the plane of incidence. 



