240 



PHYSIOLOGICAL CHEMISTRY. 



attachments for the purpose is required. Several distinct methods have 

 been applied for the purpose, but the methods now followed involve a 

 simple direct comparison between light which has been weakened by pass- 

 ing through a blood solution and light passing into the spectroscope 



directly. Such a comparison is 



"0 c Q) 1 easily made by means of instru- 

 ments with double collimator slit 

 as first introduced by Vierordt. 

 The arrangement of the appa- 

 ratus made by Kruess is shown 

 in Fig. 13, while the double 

 collimator slit and ocular and 

 reading scale are shown in Figs. 

 16 and 17. 



The method of measurement 

 depends on the principle that 

 there is a simple relation be- 

 tween the amount of light ab- 

 sorbed by a solution and the 

 concentration, that is the number 

 FIG. 1 6. Symmetrical double slit for of absorbing molecules in the 

 the absorption spectroscope. same. By rinding therefore the 



fraction of the original light 



absorbed we can arrive at the amount of absorbing substance in solution. 

 The loss of light in passing through solutions of increasing concentration 



FIG. 17. Ocular and reading scale of the Kruess spectrophotometer. 



foUows the law worked out by Lambert for the loss in passing a series 

 of glass plates of same thickness and color. Each new layer absorbs the 



