THE VISUAL PIGMENTS 



extract is introduced into a glass or quartz optical cell which is 

 bounded by plane, parallel faces (Fig. 1.5). If the intensity of Hght 

 which enters the first surface, A A, of the visual pigment solution is 



Parallel 

 beam 

 of 

 light 



->■ 



->■ 



5i k 



dl 



>- 

 >- 



Fig. 1.5. Section of cell used in the measurement of optical densities of 



solutions. The solution, of thickness /, may be regarded as a pile of an 



infinite number of discs of thickness dl. 



/inc, and the intensity of light leaving the second surface, BB, is /trans, 

 then the hght absorbed by the solution is, 



/abs ^= /inc ^trans 



and the percentage of light absorbed by it is, 



/abs 



X 100 



inc 



X 100 



If the percentage absorption is obtained for light of various wave- 

 lengths, absorption spectra can be constructed. Examples are shown 

 in Fig. 1.6 (A) for pure visual purple solutions of different concentra- 

 tions. In Fig. 1.6(B) these same curves have been replotted as 

 percentages of their respective maxima. From these it is clear that 

 absorption bands broaden with increasing concentration. This is to 

 be expected for, if a solution absorbs nearly all hght at A^ and, say, 



14 



