THE VISUAL PIGMENTS 



equations for first and second order thermal reactions, the photo- 

 sensitivity corresponding to the reaction velocity constants. Equa- 

 tion (17) contains no concentration terms. Consequently the photo- 

 sensitivity can be measured in solutions of unknown concentration. 

 This makes the method of photometric curves of particular value in 

 cases Hke that of visual purple. 



The method of photometric curves is very sensitive to the presence 

 of consecutive reactions and other complications and where these 

 occur the plot of log^o /</(// — Q against t does not yield a straight 

 line. The photochemical reaction in the bleaching of visual purple is 

 the formation of transient orange. This substance then decomposes 

 thermally to indicator yellow. At temperatures of 20°C and above 

 this conversion is extremely fast. At the low hght intensities used, 

 the photochemical change to transient orange is the rate-determining 

 reaction, all transient orange being removed (by thermal decomposi- 

 tion to indicator yellow) almost immediately after it is produced. 

 This secondary process does not interfere, therefore, and the reaction 

 proceeds as though visual purple were directly changed into indicator 

 yellow. Straight lines were consequently obtained when log /J(/y. — 

 I^ was plotted against time, and the slope-compensating factor <^, 

 depended only on the internal filter effect of indicator yellow. Below 

 20°C and above 5°C the conversion of transient orange to indicator 

 yellow proceeded at a rate comparable with that of the photochemical 

 change. Within this temperature range, therefore, the method of 

 photometric curves broke down. At 5°C transient orange was fairly 

 stable and its rate of conversion to indicator yellow was neghgible in 

 comparison with the photochemical reaction. Consequently when 

 log IJilf — It) was plotted against time, straight lines were once 

 again obtained. The reaction at this temperature was simply the 

 conversion of visual purple into transient orange, and (/>, the slope- 

 compensating factor, depended on the internal filter effects of tran- 

 sient orange. 



The maximum photosensitivity (ay). Using a bleaching Hght of 

 wavelength 506 m/.< (close to the Amax for visual purple), dartnall, 

 GOODEVE and lythgoe (1936, 1938) carried out a number of 

 experiments under widely different conditions of pH (6-8-9-2) and 

 temperature (5°-60°C). At the higher temperatures, allowances 

 were made for the effect of thermal decomposition. In all cases, save 

 for the range below 20°C and above 5°C, straight lines were obtained 



76 



