286 PHOTOCHEMISTRY OF VISUAL PURPLE. II 



gated independently; the values for the temperature coefficients of 

 the other reactions are 2.5 (p, = 19,680) and 13.5 {[x = 48,500) respec- 

 tively. An analysis of a similar hypothetical condition has been 

 made by Osterhout (1917) which shows the relation of the total 

 temperature coefficient of a process to the temperature coefficients 

 of the individual reactions composing it. 



2. In the first paper of this series (Hecht, 1920-21, c) it was shown 

 that visual purple solution, so prepared as to give an irreversible 

 photochemical reaction, is bleached by light according to the kinetics 

 of a monomolecular reaction. This in itself would seem to indicate 

 that the reaction as a whole is a simple photochemical transformation. 



Experiments with intermittent as compared with constant illumi- 

 nation supported this conclusion. Within the limits of experimental 

 error it was not possible to demonstrate the presence of any initial 

 induction period or of an after effect in the course of the reaction. 

 The presence of either or both of these irregularities frequently 

 bespeaks a complexity in the make-up of the reaction (Mellor, 1904, 

 p. 116). 



The evidence therefore indicates that the bleaching of visual 

 purple is a comparatively simple process. This is further confirmed 

 by the following experiments on the effect of temperature. 



II. 



Visual purple solutions are prepared from the dark-adapted retinas 

 of frogs by the technique previously described. ^ The changes pro- 

 duced by light are followed colorimetrically by comparison with 

 concentration standards. These are made by combining bleached 

 and unbleached visual purple in varying proportions so as to give a 

 series changing by 10 per cent steps. The experimental solutions 

 and the standards are manipulated in capillary tubes arranged to 

 give equal depths of solution. For the composition and preparation 

 of the standards and for the methods generally, the reader is referred 

 to the previous article in these studies (Hecht, 1920-21, c) where 

 full details will be found. 



^ In making solutions of visual purple, pure bile salts must be used. I am 

 indebted to Dr. S. Morgulis for furnishing me with a supply of these salts 

 purified by himself. 



