THE VISUAL PIGMENTS 



general it was found that when the bleaching light was of any 

 wavelength between 400 and 590 m« the bleaching rates were the 

 same at 32°C as at 2°C, but that when it was of longer wavelength, 

 i.e. between 590 and 750 m/i, there was a marked dependence on 

 temperature. The ratios of the bleaching rates at 32°C and 2°C (Qso) 

 are plotted against wavelength in Fig. 3.11. 



As Fig. 3.1 1 (B) shows, the temperature dependence of the bleach- 

 ing rate begins at c. 590 m^a. The size of the energy unit (quantum) 

 for this wavelength corresponds to 48,500 calories per 'mole of 

 quanta.' This is quite close to the activation energy, 44,000 calories 

 per gram-molecule, found by lythgoe and quilliam for the purely 

 thermal reaction. Up to the wavelength of 590 ma therefore the 

 energy of the quantum is adequate but at longer wavelengths — for 

 which the quantum energy falls short of the activation energy — a 

 contribution must be taken from the thermal energy stores of the 

 molecule. 



The question therefore arises whether only those molecules which 

 have a sufficient internal energy to make up the deficit can absorb an 

 'inadequate' quantum, or whether any molecule can — in which case 

 some quanta will be absorbed to no purpose, with consequent 

 reduction in the quantum efficiency. 



To settle this question, allowance must first be made for the 

 changes in extinction with temperature. ST. george measured the 

 density spectra, up to 620 m^, of both cattle and frog visual purple 

 solutions at various temperatures between 23°C and — 100°C. No 

 measurements were made at 32°C but it was assumed that the ratios 

 of the extinctions at 32°C to those at 2°C would be given sufficiently 

 closely by the ratios at 25°C to those at — 5°C. The logarithms of 

 these ratios, when plotted as a function of wavelength gave a straight 

 line between 560 and 620 mju. ST. george therefore extrapolated 

 this line to 750 m^ to obtain estimates of the extinction ratios at 

 those wavelengths where the density of the visual purple was too 

 small to be directly measured. 



In Fig. 3.11 (A) the extinction ratios so obtained are compared with 

 the observed bleaching rate ratios. Although the general trend of 

 variation with wavelength is the same for both, ST. george con- 

 sidered that the bleaching rate ratios were significantly higher than 

 the extinction ratios. In this case 'the internal energy of the rhodop- 

 sin molecule is a more critical factor in bleaching than it is in the 

 process of light absorption. Some of the molecules which absorbed 



8S 



