698 VERHOEFF AND BELL. 



more transparent screens. In the case of sunlight the exposures 

 were from one-fourth second to one and one-half minutes, and the 

 resulting burns were always severe, the retinal tissue being actually 

 coagulated as will be described. In the case of the magnetite arc the 

 exposures w^ere from ten minutes to one hour and the burns were much 

 less severe. In addition to these, heat effects involving the pigment 

 epithelium alone were obtained in two experiments with the quartz 

 double lens system (Exps. 88 and 89) in each of which a large area of 

 the fundus was illuminated. One of these was in the case of an 

 aphakic eye, and the other in a case of exposure without a water filter. 

 The significance of these experiments in connection with the questions 

 of eclipse blindness and allied phenomena is discussed elsewhere 

 (page 720). 



That the severe effects produced by concentrated sunlight were due 

 to heat was obvious from their histological appearances and from the 

 fact that the light intensity at the focus was found in all cases to 

 be sufficient quickly to ignite a match or piece of paper. That the 

 relatively slight effects produced by the magnetite arc were also due 

 to heat, was obvious from the fact that only the pigment epithelium 

 and outer retinal layers were affected and sometimes the pigment 

 layer alone. If the effects had been due to the abiotic action of light 

 the inner nuclear layer and ganglion cells would necessarily have been 

 equally or even more greatly affected. Moreover, as we have already 

 shown, when the corneal epithelium and lens epithelium were exposed 

 to light of greater intensity and shorter wave lengths than was the 

 retina in these experiments, and for a much longer time, no changes 

 were produced in them. Thus in Experiment 53 a heat effect in 

 the retina was obtained after 12 minutes exposure to light passing 

 through the lens of the eye, that is, to waves longer than 330 /x/x, 

 whereas in Experiment 85 no effect was produced on the cornea after 

 an exposure of if hours to light of greater intensity containing wave 

 lengths as short as 315 juju. This is easily explicable on the assumption 

 that the retinal changes under consideration were due to heat, since 

 the cornea and lens must each absorb a far less proportion of visible 

 and infra red rays that reach them than does the pigment epithelium 

 of the retina. On the other hand it is absolutely inexplicable on the 

 assumption that the retinal changes were due to abiotic action, since 

 it is inconceivable that the corneal and lens epithelium would be un- 

 affected by abiotic action of light sufficient to produce nuclear frag- 

 mentation in the outer muclear layer and pigment epithelium. The 

 character of the histological changes clearly indicates that the heat 



