Absorption Spectra. 229 



Seeing, therefore, that the spectrum of an absorbing body- 

 is so profoundly modified by the amount or thickness of 

 the absorbing medium, and seeing that we are acquainted 

 with no means of always securing the same amount of 

 colouring matter for observation, it must be admitted that 

 absorption spectroscopy can never compare with radiation 

 spectroscopy, as a reliable means of investigation, so long as 

 the recorded results of observation are influenced by this 

 variable factor, the amount of the absorbing material. At 

 one time it was hoped that by means of the spectroscope we 

 might be able to distinguish the blood of one animal from 

 that of another, and so render important service to medico- 

 legal investigation by discriminating such blood stains, but, 

 hitherto, no such result has been obtained, nor, with the 

 present system, is it likely to be. The same applies to 

 chlorophyll and other absorbing bodies. 



Since, then, we cannot ensure constancy in the amount 

 of absorbing medium, what is required is some method 

 which shall give uniform results which are quite independent 

 of such amount, and the object of my communication is to 

 introduce to your notice such a method. In giving con- 

 sideration to this subject it occurred to me, that, if we could 

 by any means select that element of an absorption band 

 which is common to all thickness of an absorbing medium, 

 and record this primitive line only, instead of the variable 

 broad band, we should have a recorded result which would, 

 of course, be constant for all amounts of such medium. 

 Now, since all media, if sufficiently thick, are opaque 

 for all rays of light, therefore, the thinnest film of any 

 medium must to some extent exert an absorbing effect on 

 every ray of light, but not to the same extent, and it is 

 therefore in this degree of absorption that we have our 

 essential factor, and the question resolves itself into — What 

 is the line of greatest absorption in any band ? because 

 this is the line of greatest absorption of any amount of 



