346 INTENSITY AND PHOTORECEPTION 



sen and Roscoe (Hecht, 1919-20, c). This minimum photochemncal 

 effect results from the application of 5.62 meter candle seconds of 

 energy, the intensity being inversely proportional to the time. Now 

 that we know the individual photochemical effect of these two vari- 

 ables, we may determine whether their reciprocal relation obtains in 

 the application of energy quantities greater than the minimum of 

 5.62 units. If the intensity is maintained constant and the time 

 varied, the photolytic effect is directly proportional to the time 

 (Hecht, 1918-19, a). This applies within an average range of approxi- 

 mately 50 meter candle seconds. If the Reciprocity Law holds for this 

 range of energy values as well as for the minimum, the relation be- 

 tween the intensity factor and its photochemical effect should be the 

 same, within the experimental error, as that found for the time fac- 

 tor. The photolytic effect of the light should thus be a linear function 

 of its intensity factor within a range of 50 units of energy. 



The broken line in Fig. 2 shows that this is true. The first three 

 points in Fig. 2 cover a range of 408 meter candles. This gradation 

 of intensity at an exposure of 0.133 second gives a variation of 54 

 units, similar to the range covered by the time factor. Within these 

 54 units the photochemical effect of the light is very obviously a linear 

 function of the intensity, as the straight line in Fig. 2 shows. We 

 may then conclude that the Reciprocity Law applies to the photore- 

 ception of Mya not only for the minimum energy requirement of 5.62 

 meter candle seconds, but for a range of 50 meter candle seconds as 

 well. 



The energy relations of the photoreceptor process therefore form a 

 consistent scheme of things. This brings increased confidence in the 

 results themselves as well as in their interpretation. 



VII. 



The investigation of the effect of intensity on the mechanism of 

 photic sensitivity, with which this paper has been concerned, is of 

 more than immediate interest. Most of the data on the photorecep- 

 tion of animals, particularly of higher vertebrates, are given in terms 

 of intensity. Such data have heretofore been refractory to anything 

 but a superficial treatment. 



