352 



SCIENCE 



[N. S. Vol. LIII. No. 1372 



period reaction is all set and ready to go, 

 and requires only that the light change 8 into 

 P and A so that the latter can catalyze the 

 transformation of L into T, which is the end- 

 product of the sensory process. The whole 

 photosensory mechanism may then be summed 

 up in the two reactions 



S^P + A; L\\P + A\\-^T, 



in which the symbol 1 1 P + A 1 1 means catal- 

 ysis by one or both of the precursor sub- 

 stances. The first of the two reactions occurs 

 during the sensitization period; the second 

 during the latent period. 



This hypothesis of photoreception is rather 

 concrete. The concreteness of the conceptions 

 has however proved a useful tool in the acqui- 

 sition of knowledge in this field. Time does 

 not permit the description of experiments 

 designed to test the hypothesis in numerous 

 ways. I can, however, mention just a few 

 to indicate its fruitfulness. 



The latent period is assumed to be a simple, 

 chemical reaction, perhajjs as hydrolysis or an 

 oxidation. Its behavior with the temperature 

 should therefore follow quantitatively the rule 

 deduced by Arrhenius for the relation be- 

 tween the velocity constant of a reaction and 

 the absolute temperature. This means more 

 than a mere determination of the tempera- 

 ture coefficient for 10 degrees ; it means a con- 

 tinuous relationship between temperature and 

 velocity, following certain theoretical con- 

 siderations. Experiments showed that the re- 

 action L—^T follows this prediction accur- 

 ately, and that the value of the constant, 

 ju, = 19,680, for our reaction is in accord \vith 

 those usually found for hydrolyses, saponifi- 

 cations, etc., in pure chemistry. 



Another test concerns the interrelations be- 

 tween the exposure and the latent period. I 

 have mentioned that the velocity of the latent 

 period reaction is directly proportional to the 

 exposure {i), provided the intensity (7) is 

 kept constant. This may be written 



V = kit 



If now we keep the time of exposure con- 

 stant and vary the intensity we find that 



V=hlogI 



or that the' velocity is proportional to the 

 logarithm of the intensity. Ordinary mathe- 

 matical reasoning indicates that if we com- 

 bine these two equations — which means ex- 

 perimentally that we vary simultaneously 

 both the time and the intensity — it should be 

 true that 



V = kt\ogI. 



Experiments prove that this expected relation- 

 ship indeed holds good. 



Still another and perhaps more significant 

 application of the proposed hypothesis has 

 been made. This concerns the dark adapta- 

 tion of the human eye. A careful analysis 

 of the data of dark adaptation in terms of 

 the principles discovered in these investiga- 

 tions has shown that dark adaptation and 

 protoreception in the human retina are fun- 

 damentally similar in principle to the process 

 in. Mya and Ciona. As a result there has 

 been opened up a new field of investigation in 

 retinal photochemistry which may some day 

 enable us to possess a reasonable theory of 

 vision. Selig Hecht 



Physiological Laboratort, 

 Creighton Medical College, 

 Omaha, Neb. 



THE MECHANISM OF INJURY AND 

 RECOVERY OF THE CELLi 



So.ME of the fundamental ideas of biology 

 are extraordinarily difficult to analyze or 

 define in any precise fashion. This is true 

 of such conceptions as life, vitality, injury, 

 recovery and death. To place these concep- 

 tions upon a ihore definite basis it is neces- 

 sary to investigate them by quantitative 

 methods. To illustrate this we may consider 

 some experiments which have been made upon 

 Laminaria, one of the common kelps of the 

 Atlantic coast. 



1 Address for the Symposium on General Phys-iol- 

 ogy at the meeting of the American Society of 

 Naturalisfts, December, 1920. 



