THE VISUAL PIGMENTS AND THEIR PHOTOPRODUCTS 



The facts were, however, more complex than any of these workers 

 thought and it was not until their observations had been extended and 

 given quantitative expression by lythgoe that some understanding 

 was obtained of the sequence of reactions taking place when visual 

 purple is exposed to light. Even to-day, however, these problems are 

 not completely settled. 



LYTHGOE (1937) soon recognized that there were two distinct 

 phenomena embraced by the term 'visual yellow.' He divided a 

 solution of frog visual purple into two samples, one of which was 

 cooled in ice and the other warmed to about 30°C. On exposing 

 them both to daylight, the cold solution bleached to a deep vermil- 

 lion-orange colour, the warm solution to a pale yellow. The chilled 

 solution remained orange for a considerable time at 0°C but, on 

 warming became pale yellow like the other. This simple experiment 

 suggested that by exposure to light, visual purple is first changed into 

 an unstable initial photoproduct ('transient orange') which then 

 fades — at a rate depending on the temperature to a new, and much 

 more stable substance, 'indicator yellow.' For the certain observa- 

 tion of the transient orange stage a low temperature is necessary, 

 though it can easily be seen at room temperature if the visual purple 

 is quickly destroyed by exposure to Hght of high intensity. 



Since, under ordinary conditions, transient orange makes but a 

 fleeting appearance it has passed unnoticed by many workers, 

 particularly those who have relied on the relatively slow measure- 

 ments of density spectra, rather than on the evidence of their eyes. 

 Such v/orkers have accordingly used the term 'visual yellow' to 

 describe the comparatively stable end product into which transient 

 orange passes — lythgoe' s indicator yellow. 



Indicator yellow, as its name implies is an acid-base indicator. It 

 is a deep chrome yellow colour in acid solution and pale yellow in 

 alkaUne solutions. These colour changes are reversible, and either 

 form of indicator yellow can be converted to the other by mere 

 adjustment of the pH. 



THE INITIAL PRODUCT OF BLEACHING 



Transient orange. The density spectrum of transient orange was 

 measured by lythgoe and quilliam in 1938. With their apparatus 

 the measurement of density spectra was a slow process. In order to 

 'catch' the transient orange, therefore, they carried out the experi- 

 ments at 3°C. Even at this temperature, however, the rate of fading 



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