i86 Proceedings. 



"Absorption Spectra and a method for their more accurate 

 determination." He pointed out that the value and im- 

 portance of radiation spectra are dependent on the constancy 

 and accuracy of their spectral lines. In the case of absorp- 

 tion spectra, as observed in the ordinary way, no such 

 constancy exists, the position of the absorption bands 

 varying with the strength or thickness of the absorbing 

 medium, and disappearing altogether when this is weak or 

 thin. The width of the bands, moreover, is thus rendered 

 unsuitable for comparison. By employing wedge-shaped 

 absorption cells, however, wedge-shaped absorption bands 

 are produced ; and by ascertaining the point of convergence 

 of the sides of these, the point of maximum absorption is 

 obtained, and from this the line of maximum absorption. 

 This may be termed a primitive absorption band. Evidence 

 was adduced to show that the position of these primitive 

 bands was constant for all strengths or thicknesses of the 

 absorbing media, and for any angle of a hollow prism con- 

 taining such media. A primitive spectrum consisting of 

 these bands might therefore be accepted as proof of the 

 identity of the body producing it. Diagrams of the 

 spectrum of blood under the varying conditions necessary 

 to prove the truth of these results were exhibited, and 

 Dr. Hodgkinson stated that he had satisfied himself that 

 the accuracy of the method approched that of radiation 

 spectroscopy, and that the minutest trace of blood would 

 be sufficient for observation. In the discussion which ensued, 

 the possible application of the method to criminal and 

 medical investigation, as in the cases of poisoning and blood- 

 stains, was referred to. 



