V. J. WULFF 223 



and the second is a low energy flash (14 joules) to photograph the absorj)- 

 tion spectnnn on a spectrograph plate. Control spectra were obtained by 

 using an inverted flash sequence, i.e. the low energy analytical flash was 

 fired first and the exciting flash last. The spectra on the plates were then 

 subjected to densitometry and difl"erence spectra were constructed from 

 the comparison of experimental and control spectra. Any one difference 

 spectrum represents density changes in one absorption spectrum when 

 compared to a control spectrum. 



Figure 1 illustrates the absorbance changes which occur in rhodopsin 

 solutions in a 5-cm absorption cell during and following flash excitation. 

 During the flash excitation rhodopsin exhibits two new absorption bands, 

 one in the red with a maximum at 600 n^ and one in the blue with a maxi- 

 mum at 425 m/i,. These bands are short-lived, having a duration less than 

 200 ixsec. Bleaching of the rhodopsin solution is evident 75 /^sec. after 

 onset of the exciting flash and progresses with time, reaching a maximum 



about 5 msec, after excitation at 28.5°C. The decrease in absorbance of 

 rhodopsin following flash excitation is maximal at 530 mix. The difference 

 spectrum of rhodopsin an hour after excitation shows a maximal decrease 

 in absorbance at 500 m/x. The discrepancy of 30 mp. in the bleaching maxi- 

 mum may be caused by absorption bands of intermediates in the bleach- 

 ing process. 



Lowering the temperature of the rhodopsin solutions has little effect 

 on the transient absorption Ijands but markedly slows the bleaching proc- 

 ess. Table 1 gives the changes in absorbance of rhodopsin during the first 

 millisecond following flash excitation at 9° and 22°C. Note that both the 

 rate and extent of bleaching is markedly depressed at the lower tempera- 

 ture. 



EFFECT OF TEMPERATURE ON RETINAL ACTION POTENTIAL 



Temperature also has marked effects upon two characteristics of the 

 photoreceptor response, the latent period and the magnitude of the retinal 

 action potential. 



Tiie temperature dependence of the latent period was first demonstrated 

 by Hecht (5) employing the light-initiated siphon withdrawal reflex of 



