EFFECTS OF RADIANT ENERGY ON THE EYE. 739 



save by an impossibly long exposure with accurate fixation. These 

 experiments were at far shorter distances and consequently of enor- 

 mously greater intensities than any which could be found with such 

 sources as illuminants, and we may hence conclude that so far as 

 general effects of thermic energy are concerned no source used for 

 illuminating purposes is capable under working conditions of produc- 

 ing any observable deleterious results. 



Experiments with the arc lamp are crucial because this source is 

 nearer to being a point source than any illuminant of similar power 

 and hence gives much sharper concentration of energy in the image. 

 It is with this concentration in the image that the possible damage to 

 the retina is concerned. Diffuse sources which do not come to a 

 definite focus may be entirely neglected for this particular purpose. 

 Low temperature sources of which most of the energy lies in the ex- 

 treme infra red are even less effective in concentrating energy upon 

 the retina, because the eye never focusses such wave lengths on the 

 retina except as a diffuse spot. 



The only sources from which there seems to be any material danger 

 from the standpoint of thermic effects are certain very powerful high 

 temperature sources used in the arts, such as heavy arcs used for 

 welding purposes, furnaces, electric or other, where there is customarily 

 great concentration of energy, and such purely accidental phenomena 

 as short circuits. Perhaps some of the very powerful arcs used in 

 searchlights might be included in this class of the possibly dangerous. 

 Ordinary discretion in avoiding disagreeably powerful lights or suita- 

 bly shading the eyes from them should avert easily all real danger 

 from any of these sources of radiation, save the short circuits which are 

 accidental rather than ordinary risks. Certainly no sources used for 

 lighting purposes can be classified as dangerous from the standpoint 

 of thermic effects. 



We wish particularly to emphasize the fact that so far as any possible 

 temporary or permanent injury to the retina is concerned such action 

 must depend on the concentration of energy in the image. Conse- 

 quently, extended sources of moderate intrinsic brilliancy are to be 

 preferred to intense sources. Hence it is desirable to protect all 

 sources naturally of high intrinsic brilliancy by diffusing globes. 



As regards dangers of injury to the eye from light radiation as such, 

 our experiments indicate that it has been very greatly exaggerated 

 as regards its pathological possibilities. It is undoubtedly true that 

 brilliant sources of light are disagreeable and that they produce 

 unpleasant effects in temporary scotomata, disturbance of color 



