20 IV. B. Cartmel 



ferential absorption of two films of diflFerent thickness or by 

 making a separate determinative of the reflection and subtracting 

 it. Preparations were made to measure the reflection, but this 

 part of the work was not finished, though I expect to continue 

 this work and measure the reflection. Not having the reflection 

 measurements, I must be content with computing them. The 

 effect of an error in the reflection upon the final values of the 

 absorption coefficients will be greatest within the absorption 

 bands, and the lo per cent change in the reflection only changes 

 the absorption coefficient i per cent. 



For computing the reflection from an absorbing substance into 

 air, Cauchy's theory leads to the following formula: 



while the electromagnetic theory leads to 



R. 



which may be seen to be identical with the previous formula when 

 we remember that in Cauchy's theory the quantity k is identical 

 with p-k of the electromagnetic theory. For the reflection at the 

 interface between an absorbing substance and a transparent sub- 

 stance whose indices of refraction are ju, and /*' respectively, the 

 electromagnetic formula becomes 



According to this formula, the reflection is the same on either 

 side of the interface between the media, while with Cauchy's 

 formula the reflection on the two sides will, in some instances, be 

 different, which would lead to the loss by reflection being diflFer- 

 ent, according to whether the light went through the glass plate 

 and then through the fuchsin or through the fuchsin first, an 

 effect which the writer has been unable to observe. The values 

 of the reflexion as computed in this way are plotted in plate III. 



120 



