EFFECTS OF RADIANT ENERGY ON THE EYE. 723 



appeared not desirable to eliminate the possibility of specific injury 

 due to such rays we employed in part of the experimental work as a 

 substitute of the mirror in the concentration of the solar energy a 

 quartz lens 12 cm. in diameter and 25 cm. focal length mounted on 

 an adjustable stand so that we could work directly in its focus. 

 A priori the climical evidence is strongly against any definite pathologi- 

 cal effects due to the ultra violet radiation as such. Numerous cases 

 are recorded in which typical eclipse blindness has been produced 

 through ordinary spectacle lenses, through glass insufficiently dark, 

 and through opera glasses and the like, in all of which cases the abiotic 

 rays are, as we have already shown, cut off. Even very small thick- 

 nesses of colored or even clear glasses are sufficient completely to ab- 

 sorb these rays, which moreover are always cut off by the lens so that 

 they cannot reach the retina where the lesions are found. This is in 

 accordance with the conclusions reached by Parsons ^^^ in analyzing 

 the evidence at hand. Attempts have been made by several investi- 

 gators, notably Birch-Hirschfeld, to eliminate the infra red rays also 

 by the use of thick water cells and other absorbing media, but these 

 attempts so far as experiments with solar light are concerned are fu- 

 tile, because, as a glance at Figure 5 (page 705) will show, the greater 

 portion of the solar energy lies entirely within the visible spectrum with 

 its intense maximum in the blue or green according to the effect of the 

 atmospheric absorption, so that for solar radiation it is the light rays 

 which are thermally effective, the energy radiation in the ultra violet 

 and infra red being relatively insignificant. Our experiments show 

 with the utmost distinctness that the effects known as eclipse blind- 

 ness are wholly thermic, due to the intense concentration of the solar 

 energy upon the retina by the refracting system of the eye itself 

 forming an image of destructive energy intensity, the amount of which 

 we have already computed in considering the energy concentrated in 

 images. It is only the briefness of the casual fixations of the sun, and 

 the great reduction of the size of the pupil in response to the intense 

 illumination, that prevents the very common occurrence of such 

 injuries. In the observation of an eclipse the patient is tempted to 

 dangerously long fixation and the necessary results follow. 



With long fixation the typical retinal lesions of eclipse blindness 

 may be produced by sources of moderate intensity. For instance in 

 our experiment No. 53 an exposure of twelve minutes was made on 

 the eye of an albino rabbit with the single quartz lens system and the 

 magnetite arc as source, through the 5 cm. quartz water cell. Now 

 from experiments previously made on the radiation from the magnetite 



