. SELIG HECHT 373 



groups of experiments shown in the two figures were made at intervals 

 of about 3 weeks, with different sources of light, and, of course, with 

 different animals. 



There can therefore be no doubt that equations (3) and (5) are true 

 experimentally. The photochemical effect of light in the photosensory 

 stimulation of Mya is then proportional to the product of the loga- 

 rithm of the intensity of illumination into the time during which the 

 light acts. This result supports previous investigations with Mya (cf. 

 especially Hecht, 1919-20, c), and strengthens the hypothesis proposed 

 to account for its photosensory responses. 



SUMMARY, 



In its photosensory effect, the action of light depends on two vari- 

 ables, — intensity and time. If the intensity alone is varied, the pho- 

 tochemical effect is proportional to the logarithm of the intensity. 

 If the time alone is varied, the effect is proportional to the time. Ex- 

 periments here reported show that when both the intensity and the 

 time are varied, the photochemical effect is equal to the product of 

 their separate activities: E = kt log 7. These results furnish the 

 means of expressing directly the relation between the intensity of 

 illumination and the reaction time of Mya. 



BIBLIOGRAPHY. 



Hecht, S., (a) The nature of the latent period in the photic response of Mya 

 arenaria, J. Gen. Physiol., 1918-19, i, 657; (b) The photochemical nature of 

 the photosensory process, 1919-20, ii, 229; (c) Intensity and the process of 

 photoreception, 337. 



Laurens, H., and Hooker, H. D., Jr., Studies on the relative physiological value 

 of spectral lights. II. The sensibihty oiVolvox to wave-lengths of equal 

 energy content, /. Exp. Zool., 1920, xxx, 345. 



