THE VISUAL PIGMENTS 



a quantum of red light, but which had too Httle thermal energy 

 before absorption to become fully activated by the quantum, do not 

 bleach; and the quantum efficiency consequently falls.* 



ST. GEORGE apparently assessed the photochemical reaction rates 

 by measuring the slopes {dijdt) of the transmission/time curves. 

 These were approximately linear over the ranges studied (Fig. 3.10). 

 Now the kinetics of a photochemical reaction is given (p. 74) by the 

 equation 



L , ay/ 



log = -— . t + constant 



If — h A. 



Differentiating both sides with respect to time we obtain, 



If ^^A ^y^ 



(I, - i,)it ' at "^ A 



Thus the slope (dijdt) at any point of a transmission/time curve must 

 be multiplied by the factor /^/(/^ — 7^)/^ in order to become a measure 

 of the reaction rate (ay). Consider, for example, the data for 678 m/x 

 in Fig. 10. The factor by which the slope of the 1^° line should be 

 multiplied is 100/(100 - 20)20 = 100/1,600 while for the 32° hne 

 the factor is 100/(100 - 27)27 = 100/1,970. Thus the ratio of the 

 slopes of the lines should be multiplied by 1,600/1,970, giving a value 

 for the ratio about 20 per cent less than that calculated by ST. george. 

 Such corrections are in the direction to give a better agreement 

 between the photosensitivity and extinction ratios. 



In view of this and the fact that the values for the extinction ratios 

 in the most critical region v/ere obtained by an extrapolation, it 

 seems unnecessary to look any further for an explanation of the 

 observed variation with wavelength of the photosensitivity ratio 

 than to the similar variation of the extinction ratio. This suggests 

 that an 'inadequate' quantum would not be absorbed unless the 

 absorbing molecule could supply the deficit energy. All quanta 

 absorbed would, therefore, be photochemically active. 



3. MOLECULAR WEIGHT 



THE WEIGHT OF THE CHROMOPHORE CARRIER 



Visual purple is a chromoprotein. Like haemoglobin, ovoverdin, 

 and other members of this class of coloured proteins, its molecule 



90 



