'2'2{j 3Ir. W. B. Cartniel on the Anomalous 



opal glass which would cause a loss of 95 to 98 per cent., 

 and the absence of Nicol prisms which would cause a further 

 loss, makes it singularly well adapted for work on strongly 

 absorbing substances. 



The diminution in intensity caused by transmission through 

 the fuchsin-lilm is due to two factors, reflexion from its sur- 

 faces and absorption within the film. To determine the 

 absorption it is necessary either to eliminate the reflexion by 

 measuring the differential absorption of two films of difterent 

 thickness or by making a separate determination of the re- 

 flexion and subtracting it. Preparations were made to 

 measure the reflexion, but this part of the work was not 

 finished, though I expect to continue this work and measure 

 the reflexion. 2s ot having the reflexion measurements I must 

 be content with computing them. The effect of an error in 

 the reflexion upon the final values of the absorption-coeflicients 

 will be greatest within the absorption-band, and even there 

 a ten per cent, change in the reflexion only changes the 

 absorption coeflicient one per cent. 



For computing the reflexion from an absorbing substance 

 into air Cauchr's theory leads to the following formula : — 



while the electromagnetic theorv leads to 



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 fiK of the electromagnetic theory. For the 

 reflexion at the interface between an absorbing substance and 

 a transparent substance whose indices of refraction are a and 

 fju' respectively, the electromagnetic formula becomes 



According to this formula the reflexion is the same on either 

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

 formula the reflexion on the two sides will in some instances 

 be difi:erent, which would lead to the loss bv reflexion beino- 

 different according to u-hether 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 wav 

 are plotted in Pi. X. fig. 3. 



If /'i, /oj and ?'3 represent the reflexion at the fuchsin -air, 

 fuchsin-glass, and air-glass surfaces respectively, and I^ the 



