THE VISUAL PIGMENTS 



This indicated that there was no induction or after-effect associated 

 with the bleaching process. 



In a second investigation (hecht, 1921), the effect of temperature 

 was examined. The same technique was used except that the small 

 tubes were immersed in a cyhndrical glass vessel filled with water, the 

 temperature of which was kept constant to 1°C by suitable additions. 

 The intensity of the bleaching light was the same for all experiments 

 (50 metre candles). The velocity constants {k) obtained were found 

 to be unaffected by temperature, measurements being made at 5-2, 

 20-0 and 36- TC. From this hecht concluded that the mono- 

 molecular course of the bleaching represented the actual photo- 

 chemical reaction and was not, for example, a measure of some sub- 

 sequent diffusion process (which would be temperature-dependent). 



Finally hecht (1924) studied the effect of intensity of the white 

 bleaching light and found that, other things being equal, the velocity 

 constant was proportional to the light intensity. 



Before describing the modern work on the photochemistry of 

 visual purple certain fundamental concepts may be reviewed. 



PHOTOCHEMICAL PRINCIPLES 



The fundamental principle of photochemistry is the law, put 

 forward independently by von Grotthuss in 1817 and by Draper 

 in 1841 and verified innumerable times since, that only light which is 

 absorbed can cause a photochemical change. Light which is not 

 absorbed can have no effect; though it is sometimes possible for 

 fight absorbed by one molecular species to cause change in another, 

 in intimate contact with it, by a process of energy-transfer known as 

 photosensitization. Visual purple itself, in certain circumstances will 

 act as a photosensitizer. Thus kogel (1929) found that Perutz 

 lantern plates which had been converted to Agl by means of the 

 Renwick iodide-sulphite-thiosulphite solution were increased in 

 general sensitivity, and more especially in the green and orange, by 

 bathing them in a 2 per cent sodium glycocholate extract of visual 

 purple. The sensitization conferred diminished rapidly, however, and 

 had disappeared in 24 hours. In this instance the energy at longer 

 wavelengths, normally unabsorbed by the Agl emulsion, was made 

 available to it by the mediation of visual purple. In general, however, 

 the molecular species which absorbs the energy is the one in which 

 the photochemical changes occur. 



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