516 Mr. E. S. Johonnott on the Thickness of 



reflexion from the film. The other slit was that of a silvered- 

 glass photometer. 



Description of the Apparatus. — The photometer consisted 

 of a piece of plate-glass P (fig. 5), lightly silvered on one 

 side with a wedge-shaped silver film and placed behind the 



Fig. 5. 



t= — r-H Af n a K in — £ # U 



^ 



slit K. The sensitiveness was increased by placing at P' a 

 sheet of ground glass which was capable of rotation about a 

 horizontal axis. A lens, L, rendered the rays from S parallel, 

 after which they were reflected through a portion of the 

 objective of the telescope by means of a pair of mirrors, R. 

 Two nicols, N and W, were placed in front of the objective, 

 through which all light reaching the telescope from the colli- 

 mator passed. A small circular aperture, A, was placed in 

 front of the nicols so as to allow the light to pass through 

 their central portions only. Precautions were taken in 

 observations at all angles of incidence to have the reflected 

 wave-front completely cover this aperture. Another smaller 

 aperture, E, helped in properly placing the eye. 



Method of Observation. — The manner of making an obser- 

 vation was as follows : — Set the two nicols parallel; turn the 

 collimator to the angle of incidence at which the reading is 

 wanted; raise the film, F, and turn it until the image of the 

 collimator-slit is close beside that of the photometer. Then, 

 as soon as the black film is formed, adjust the photometer so 

 that both images have the same intensity. The film is then 

 removed, the collimator turned into line with the telescope, 

 and the nicol N turned about its axis until the photometer is 

 again matched. 



Theory of the Method. — The intensity of the reflected light 

 is given by the equation 



I = cos 2 6>, (1) 



where 6 is the angle through which the nicol N is turned and 

 I is one half of the light incident on the front nicol N' 

 (neglecting loss due to reflexion) . By properly setting the 

 front nicolj the observation may be made with light polarized 



