704 VERHOEFF AND BELL. 



In the case of heat, deleterious effects on the tissues must Hkewise 

 be due to chemical changes. Since, however, these changes take 

 place only when the molecules have reached a certain rate of motion, 

 under ordinary conditions a measurable time interval must elapse 

 before they begin. The length of the time interval depends upon the 

 intensity of the light and upon the rapidity with which dissipation 

 of heat occurs, and thus varies greatly under different conditions. 

 Under ordinary conditions, however, the time interval is of consider- 

 able length, so that a series of exposures does not produce a total 

 effect equal to that of a continuous exposure of the same total length, 

 and may not produce any effect at all. From a practical standpoint 

 therefore this fact constitutes a fundamental difference between the 

 abiotic effects and the heat effects of radiant energy. 



Unless light rays are absorbed by substances they can of course 

 produce no effect upon them. Thus, as we have shown, waves over 

 295 iJLfjL in length unless extremely intense have no effect on the corneal 

 stroma which is relatively transparent to them but have a markedly 

 deleterious effect on the corneal corpuscles which absorb them. It 

 does- not necessarily follow, however, that because light rays fail to 

 pass through a given substance they must produce an effect upon it. 

 For they may simply be changed into light waves of longer wave length 

 (fluorescence) or their energy dissipated in the form of heat of an in- 

 tensity too low to produce any changes. Both of these transforma- 

 tions must take place in the case of the lens of the eye since light 

 waves are constantly being stopped in it. That fluorescence actually 

 occurs in the lens is, of course, well known and easily demonstrated. 



Assuming that the abiotic action of light of given wave lengths 

 upon protoplasm is directly proportional to the coefficient of absorp- 

 tion of the protoplasm for, that wave length, Henri ^^^ and his wife 

 have determined this coefficient for egg albumin and a large number 

 of waves. The curve plotted from their results given elsewhere 

 (page 645) shows that the absorption becomes practically nil at and 

 near 310 /x/x, so that the abiotic action must be very slight here. 

 Moreover, since this method does not allow for the fact that the 

 absorbed rays produce heat as well as abiotic effects, the abiotic 

 action is undoubtedly less than is indicated by the curve of absorp- 

 tion. These results, therefore, confirm in a striking manner those 

 obtained by us by actual experiments on the cornea. 



The preceding discussion has concerned mainly the direct effects 

 of light upon the molecules of the tissues without reference to histo- 

 logical and clinical manifestations. The latter are of course, too 



