EFFECTS OF RADIANT ENERGY ON THE EYE. 679 



all those less than 400 njx in length. Now we have already shown 

 tiiat the corneal epithelium and lens capsule are not affected in the 

 slightest degree when exposed one and one-half hours to rays as short 

 as 310 AtjU even when the intensity is considerably greater than that to 

 which the retina is ever subjected in the case of any of the known 

 artificial light sources. This exposure is at least forty-five times 

 greater than that required to affect the corneal epithelium by waves 

 of 295 /X)U and less. For the retina therefore to be affected by the 

 abiotic action of light transmitted by the lens, it would have to be 

 many times more sensitive to such action than the corneal epithelium. 

 There is no reason to believe however, that this is the case, but on the 

 contrary, since the abiotic effect depends upon the amount of absorp- 

 tion of the waves, there is strong reason for believing that the corneal 

 epithelium and retina are about equally sensitive to abiotic action. 

 Assuming this to be so, these experiments show conclusively that the 

 human eye could be fixed steadily and at close range upon the magne- 

 tite arc certainly for over two hours and probably for many hours 

 without suffering damage to the retina from abiotic action. Since 

 as already pointed out the intensity of the image of a source of light of 

 such small size as that in question decreases as the square of the dis- 

 tance, the danger of injury to the retina at ordinary distances would 

 be absolutely negligible. 



In the case of the lenses of some children, Hallauer found a very 

 weak transmission band at 315 to 330 ju/x. This, however, does not 

 invalidate the application of the above argument to the case of chil- 

 dren, since we have shown that such waves are without abiotic eft'ect. 



While it seems to us that the foregoing facts prove conclusively 

 enough that the lens affords complete protection to the retina from the 

 abiotic action of light, in view of the fact that Birch-Hirschfeld claims 

 to have produced pathological changes in the retinae of normal rab- 

 bit's eyes by exposure to ultra violet light (cf . page 687), we have under- 

 taken to investigate this question by direct experiment upon the retina 

 itself. 



Such an investigation presents several difficulties. In the first 

 place it is impossible to reproduce with animals exactly the conditions 

 that obtain when the human eye is fixed upon a small intense source 

 of light. This is so because it is impossible to insure in the case 

 of an animal that the small image of the light source will always fall 

 upon the same spot in the retina during the exposure. Moreover, 

 even if this were possible it would be difficult if not impossible to find 

 with certainty such a small area on microscopic examination unless 



