406 Comparative Animal Physiology 



THE RECIPROCITY LAW. The photic rcsponsc of the clam, Mya arenaria, 

 consists of the withdrawal of the extended siphon. This response is char- 

 acterized by: (1) a threshold— a certain minimum intensity and duration 

 of illumination is necessary to produce a response; and (2) a variable latent 

 period— the time lapse between the end of the exposure and the beginning 

 of the response varies in a definite manner with the conditions of illumina- 

 tion. The latent period and exposure period are often measured together and 

 referred to as reaction time. 



Hecht'^^ determined the intensity of illumination necessary just to produce 

 a response for various durations of illumination in a maximally sensitive ani- 

 mal. The results indicate that the magnitude of the product of intensity and 

 time necessary to elicit a response was a constant equal to 5.62 meter candle 

 seconds. Expressed in mathematical notations, 



Ft=It 



where Et is the photochemical effect for a threshold response, I is the in- 

 tensity in meter candles, and t is the time in seconds. This is a form of a 

 fundamental principle in photochemistry, the Bunsen-Roscoe reciprocity law, 

 which states that the photochemical effect is equal to the product of intensity 

 and time. It should be emphasized that the above application of the reciproc- 

 ity law to the photic response of Mya is valid for only one level of response, 

 namely, the threshold level within the intensity range used. The data do not 

 indicate that a linear relation between photochemical effect and intensity-time 

 product exists for all levels of stimulation. 



TEMPERATURE COEFFICIENT OF PHOTOCHEMICAL REACTIONS. Hccht^^ de- 

 termined the radiant energy necessary just to produce a response in a max- 

 imallv sensitive animal equilibrated to different environmental temperatures. 

 Over a temperature range of 15 to 25° C, the energy required is constant, 

 and the temperature coefficients, Qio, ranged from 1.04 to 1.07. Photo- 

 chemical reactions are known to be thermostable, and these results agree with 

 this well known fact, indicating that the initial and controlling reaction in 

 photoreception is a photochemical reaction. 



DARK ADAPTATION IN THE CLAM. An increase in sensitivity to light after a 

 sojourn in darkness is common to all photoreceptor systems. This phenom- 

 enon is called dark adaptation. Hecht^^ determined the course of dark adap- 

 tation in the clam, Mya. The animal was illuminated with a constant in- 

 tensity ol light for a prolonged period. This illumination was terminated 

 suddenly and the animal was in total darkness. Immediately, the animal 

 was stimulated with a flash of light and the reaction time was obtained. 

 Soon another flash of light (of the same intensity and duration as the pre- 

 vious flash) was admitted and the reaction time again was measured. This 

 procedure was repeated several times during a 50 minute period in the dark. 

 The reaction time was found to decrease with increasing time in the dark 

 (see Fig. 116, lower curve). Hecht argued that the reaction time was oc- 

 cupied by a reaction whose velocity varied with the magnitude of the photo- 

 chemical effect; the greater the photochemical effect, the faster the reaction 

 or the shorter the reaction time. The decrease in reaction time during the 

 stay in the dark implied an increased photochemical effect elicited by the 

 constant intensity-constant duration test flashes— which in turn implied 



