670 TEMPERATURE AND LATENT PERIOD 



The duration of the latent period is determined by the velocity of an 

 independent chemical reaction 



L-^T (2) 



in which an innocuous substance {L) is changed into the stimulating 

 substance {T). The activity of this reaction is catalyzed by the 

 presence of one or both of the precursor materials {P and A) freshly 

 formed during the sensitization period (Hecht, 1919, h). A slight 

 fraction of the latent period represents the time for the conduction of 

 impulses. This process, however, is very rapid, and may be entirely 

 discounted in the comparatively large magnitudes of time concerned 

 in the latent period. 



The effect of temperature changes on the reaction time of Mya 

 can therefore be considered as causing changes in the velocity of a 

 photochemical reaction (1) and of an ordinary chemical reaction (2). 

 It is well known that photochemical reactions suffer but slight changes 

 with an alteration in the temperature. In fact, the sensitization 

 process in Mya representing this photochemical reaction varies but 

 slightly with the temperature. Data to be presented elsewhere give 

 its temperature coefficient for 10°C. to be about 1.3 or less. The 

 latent period, however, representing an ordinary chemical reaction 

 varies decidedly with the temperature. This may be demonstrated 

 by determining the latent period at different temperatures as a 

 result of a constant exposure period. Fig. 1 gives the data of one 

 such experiment in which the exposure was for 0.078 second to an 

 intensity of 400 meter candles. The latent period is the difference 

 between the total reaction time and the exposure period of 0.078 

 second. It is therefore clear that the bulk of the, variation in the 

 reaction time resulting from a change in temperature is due to the 

 alteration in the latent period. Consequently, by subtracting the 

 sensitization period from the reaction time determined as a whole, it 

 is possible to measure the exact relation between the temperature 

 and the duration of the latent period. 



The experiments thus resolve themselves into measuring at dif- 

 ferent temperatures the reaction time of an animal to a single in- 

 tensity of light. Preliminary experiments showed that about 40 

 meter candles is the right intensity to use. This gives a reaction 



