THE VISUAL PIGMENTS AND THEIR PHOTOPRODUCTS 



BERGER and SEGAL'S experiments recall the old Visual yellow' 

 controversy, ewald and kuhne reported that Visual yellow' was 

 photosensitive, a finding which kottgen and abelsdorff could not 

 confirm, because they obtained the same difference spectrum irre- 

 spective of whether they bleached visual purple solutions with long- 

 or with short-wave Hght (p. 42). May it not be that in ewald and 

 kUhne's experiments the 'transient' orange was thermally stable and 

 photosensitive (as in berger and segal's experiments); while in 



1-0 

 0-9 

 0-8 

 0-7 



f 0-6 

 i 0-5 



Oi-fJ 



400 



450 



550 



600 



500 



Wavelength in mu 



Fig. 2.11. Variation of the density spectrum of indicator yellow with 



hydrogen ion concentration. The density spectrum of the parent (frog) 



visual purple is shown by the dotted curve. 



{Lythgoe, 1937) 



KOTTGEN and abelsdorff' s, as in most other workers' experience, 

 the transient orange thermally decomposed to indicator yellow with 

 such rapidity that any light-sensitiveness it may have possessed was 

 overlooked ? Further work is clearly required to elucidate these points. 



THE FINAL PRODUCTS OF BLEACHING 



Under ordinary experimental conditions, the initial products 

 obtained by bleaching visual purple in solution are rapidly trans- 

 formed into indicator yellow. Indicator yellow is an acid-base 

 indicator, being deep chrome yellow in acid, and pale yellow in 

 alkaUne solutions. The reactions of this relatively stable substance 

 were first clearly characterized by lythgoe (1937). 



lythgoe's measurements of the density spectra of indicator 

 yellow solutions at various acidities within the range pH 5-2-10-0 

 are shown in Fig. 2.11. The density spectrum of the parent visual 



51 



