EFFECTS OF RADIANT ENERGY ON THE EYE. 689 



was placed before the eye these changes did not occur. The exact 

 length of time after exposure when the eyes were examined is not 

 stated. It also is not stated whether or not the changes could be 

 found if the eyes were removed immediately after the exposures. 



It is impossible for us to accept the findings in these experiments 

 for the following reasons. In the first place the retinal changes 

 described were widespread. To obtain a widespread illumination 

 of the retina, however, necessitates the use of a quartz lens of extreme 

 aperture. Birch-Hirschfeld does not state that he used such a lens. 

 The widespread illumination of the retina also necessitates a greater 

 intensity of illumination of the cornea and lens than of the retina. 

 This together with the fact that the lens capsule receives in addition 

 rays of much shorter wave length than can reach the retina makes it 

 inconceivable that the retina could be injured under these conditions 

 without the lens capsule also being affected. Yet Birch-Hirschfeld 

 states that in neither series of experiments was the intensity and dura- 

 tion of exposure sufficient to injure the lens. In fact the abiotic 

 intensity was so slight that the corneal epithelium was destroNcd only 

 in one experiment in which the cornea apparently became infected. 



On the other hand if we assume that Birch-Hirschfeld used no lens 

 or a lens of ordinary aperture, the retinal lesions, if any, would have 

 been circumscribed and would not often have been found by his 

 method of examining the eyes. As we shall show, with sufficient light 

 intensity small retinal lesions can be produced under these condi- 

 tions, but they are due to heat and are entirely different from those 

 described by Birch-Hirschfeld. The radiant energy used b.y Birch- 

 Hirschfeld, however, was undoubtedly insufficient to produce such 

 an effect. 



In his first series of experiments it is stated that the changes occurred 

 immediately after the exposures. This is inconsistent with an abi- 

 otic action of light, since with this there is always a latent period. 

 Thus we found that the epithelial cells of the lens capsule showed 

 absolutely no change if examined immediately after severe exposure 

 to abiotic rays. 



Birch-Hirschfeld holds that the ganglion cell changes he describes 

 represent a further stage of light adaptation. Yet he maintains that 

 they are due to the direct action of the light on the ganglion cells 

 themselves. He states that there is no reason to believe that certain 

 cells are more susceptible to ultra violet light than others, yet he found 

 changes in the retinal ganglion cells and none in the capsular epithe- 

 lium in spite of the fact, just pointed out, that the latter must have 



