678 VERHOEFF AND BELL. 



In fact under certain conditions, and witli a moderately dilated pupil 

 the intensity of the light reaching the retina will he enormously 

 greater than the same light as it passes through the cornea. For this 

 reason it will be seen that if the transmissible rays were capable of 

 injuring tissue cells, the macula of the eye might be seriously damaged 

 in spite of the fact that the cornea and lens remained unaffected. 

 This, of course, actually happens in eclipse blindness in which, how- 

 ever, as will be pointed out, the effect is due entirely to heat generated 

 in the pigment epithelium. 



There are two conceivable ways, exclusive of heat effects, in which 

 the retina coidd be injured by light. If the light were sufficiently 

 intense it might overstimulate the physiological mechanism upon 

 which the perception of light is dependent and thus lead to more or 

 less permanent impairment of this mechanism. It is obvious that 

 such an effect could not readily be produced by light of wave lengths 

 less than 400 /x/x since the latter has relatively little power to stimulate 

 this mechanism even in aphakic eyes. The other possibility is that 

 intense light might injure the cells of the retina by abiotic action in 

 the same way that light rays of short wave length injure tissue cells 

 in general. In connection with this possibility two facts previously 

 established by us must be taken into consideration, namely that 

 within wide limits discontinuous exposures to abiotic rays have the 

 same total effect as a continuous exposure of the same total length, 

 and that there is a limit below which such siunmation does not occur. 

 Thus it would a priori seem possible that if an indi\idual fixed a bright 

 source of light many times daily, serious damage to the macula might 

 result. 



The problem in regard to the retina that chiefly concerns us in the 

 present investigation may be briefly stated thus: exclusive of a heat 

 effect, can the retina of the human eye be injured by light of an}' or all 

 wave lengths that can possibly reach it through the cornea and lens? 

 In attempting to answer this question it is important first to inquire 

 whether or not the waves that are able to pass through the dioptric 

 media are injurious to tissue cells in general. If they are so injurious 

 the question is obviously to be answered in the affirmative. If they 

 are not, the question is in all probability to be answered in the nega- 

 tiv^e, but not perhaps with absolute certainty, since it is conceivable 

 that the retinal cells are more susceptible to injury by light than are 

 other tissue cells. 



It has been shown by Hallauer ^^^ and others that the adult human 

 lens always absorbs all waves less than 376 mm in length, and usually 



