\ahen exposed to Polarized Light. 183 



the polarizing angle of water. The effect, which was very 

 striking, is shown in Plate II. fig. 1. The central part, AB, 

 of the system of rings, CDEF, was without rings and colours 

 of any kind: the upper half, CD, was part of a system of rings 

 with white circumferences, and was formed by polarized light 

 incident on the film at an angle greater than the polarizing 

 angle of water; while the under half, EF, was part of a system 

 of rings with black circumferences like those seen by common 

 light, and was formed by polarized light incident on the film 

 at an angle less than the polarizing angle of water. 



The absence of rings in the middle portion, AB, was of 

 course owing to there being no light reflected from the Jirst 

 surface of the film with which that reflected from the second 

 surface could interfere ; and the reason of there being light 

 reflected from the second surface was, that the light reflected 

 from it was not incident at its polarizing angle. 



1 have elsewhere shown*, that when a film of water is laid ujtbn 



glass whose refractive index is above 1 "508, there is no angle 



of incidence upon the first surface of the film which will allow 



the refracted ray to fall upon the glass at the polarizing angle; 



and hence at every angle of incidence on the film, the refracted 



light is reflected from the glass at angles less than the polariziiig 



angle of the united media^ or less than an angle whose tangent 



m 

 is equal to — , m being the refractive index of the glass, and 



?h' that of the water. When the refractive index of the glass 

 is 1*508, the angle of incidence on the film must be 90° exactly, 

 in order that the refracted ray may fall upon the glass at the 



polarizing angle whose tangent is equal to — ^. 



Now as the portion of the coloured rings at CD, fig. 1, is 

 formed by the interference of two pencils, CA, DEB, fig. 2, 

 one of which, CA, is reflected at an angle, PCA, above the 

 polarizing angle of water, and the other, EB, at an angle be- 

 low or less than that angle; while the portion EF, fig. 1, is 

 formed by the interference of two pencils, which are both re- 

 flected at angles below or less than that angle, we may suppose 

 that in the formation of the rings with a white circumference, 

 analogous to those with a white centre, there is a loss of half 

 an undulation, while that loss takes place in the interference 

 of common light, or of two pencils reflected on the same side 

 of the polarizing angle. 



When the rings are seen at angles between 0^ of incidence 

 and 53'^ 1 1, the polarizing angle of water, they are black in 



• PhiioEophical Transactions, 1815, p. 138. 



