Dispei'sion and Selective Absorptioi of Fuchsin. 'I'll 



oriojinal intensity o£ the incident light, the effect o£ the re- 

 flexion is to make the transmitted light have the value 

 (1 — >'i)(l — /'2)(1— /-JIo? neglecting the etiect of multiple re- 

 flexions which does not enter in ^vithin the absorption-bands. 

 For those colours for which the film is transparent the effect 

 of multiple reflexion is easily allowed for to the degree of 

 accuracy possible when working with strongly absorbing 

 substances. 



Knowing the thickness o£ the film we may compute the 

 absorption-coethcient f.iK. from the following formula 



I 



I 



Io(l-/'J(l-r,)(l-.3) 



in which ^ is the ratio o£ the intensities as determined by 



fo 

 the spectrophotometer, t is the thickness o£ the film, and \ 

 the wave-length in vacuo. The photometric measurements 

 were all made upon a film 192>t^i thick. The transmission 

 o£ this film is plotted in PI. X. fig. 2. Using these values the 

 curve given in PL X. fig. 1 was computed. In the following- 

 table these values are compared with Pfiiiger^s and Walter^s 

 results : — 



Values of a^K. 



X. 



Pfliiger. 



Walter. 



Cavtmel. 



589 



0-79 



0-792 



■^(^"^ 



527 



1-22 



1-419 



1-354* 



486 



0-98 



1-168 



1-07* 



455 



0-43 



0-533 



0-56* 



* By interpolation. 



Walter used Cauchy^s formulae for the elliptic polarization 

 which is associated with metallic reflexion, and as this could 

 only be applied to those radiations which are strongly absorbed 

 by the fuchsin, he only obtained the four values given above. 

 In order to make a comparison with Walter's work Pfliiger 

 measured the same four quantities directly, using the difference 

 in transmission o£ two films of different thickness. 



In conclusion, my best thanks are due to Prof. Brace for 

 help and encouragement throughout the work, and for the 

 excellent laboratory facilities afforded me at the University 

 of Nebraska where the work was done. I must also thank 

 Prof. B. E. Moore for various courtesies extended to me, and 

 for suggestions in regard to the photometric measurements. 



National Bureau of Standards. Washington. 



Q2 



