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



apparatus had been taken down and entirely readjusted. This 

 may possibly account for the difference in the readings, 

 although it may also have been due to a difference in the 

 solutions. Some of the points lie above a smooth curve 

 through the others, which may be due to imperfect films, as 

 some difficulty was experienced in getting stable films. The 

 thickness of the films is about 6 \x\l. The temperature of the 

 room was about 23° C. 



Fk-. 6. 



OOl 



To 30 40 SO ^ ~70 3£" 



Curve C. — This was taken July 13th, with the same 

 solution as B, that had stood exposed one night. The plane 

 of polarization was perpendicular to the plane of incidence. 

 The intensity fell until the angle of incidence was 52°' 5, at 

 which the image of the collimator-slit could barely be 



