EFFECTS OF RADIANT ENERGY ON THE EYE. 741 



the retina is uninjured under these extreme conditions seems to indi- 

 cate as do our experiments that the eye is remarkably tolerant of 

 intense light even under the circumstances of exposures of great 

 severity lasting over very long periods of time. 



These results do not justify the use of powerful unscreened sources 

 near the eye, since in this condition vision becomes difficult and the 

 effects of eye-strain due to other causes than mere illumination of the 

 retina become unpleasantly in evidence. They do show, however, 

 that the eye in the process of its evolution has acquired the ability to 

 take care of itself under extreme conditions of illumination to a degree 

 hitherto deemed highly improbable, and that the effects on the 

 retina due to any exposure to intense light in the least degree likely to 

 be found in the use of practical illuminants are temporary and of no 

 pathological significance. A fortiori, there is not even a remote chance 

 of pathological effects on the structure of the eye due to light received 

 from extended surfaces of low intensity like translucent globes and 

 diffuse reflections as from paper. 



As for the ultra violet part of the spectrum to which exaggerated 

 importance has been attached by many recent writers, the situation 

 is much the same as with respect to the rest of the spectrum, that is, 

 while under conceivable or realizable conditions of over exposure 

 injury may be done to the external eye yet under all practical condi- 

 tions found in actual use of artificial sources of light for illumination 

 the ultra violet part of the spectrum may be left out as a possible 

 source of injury. All illuminants possess an easily measurable amount 

 of ultra violet radiation ranging, as one of us has already shown ^^ 

 from about 4 ergs per second per square cm. per foot candle of illumi- 

 nation in the quartz arc with the usual globe, to more than tw^enty 

 times this amount in the enclosed carbon arc shining through a quartz 

 window. Between these two lie the whole range of incandescent lamps 

 both gas and electric, the ordinary mercury arcs and the ordinary 

 Cooper Hewitt tube, flames, arc lamps of various sorts, and sunlight. 

 The last mentioned occupies an intermediate position between the 

 high efficiency electric incandescent lamps and the older incandescent 

 lamps or ordinary flames. The ultra violet in these various sources 

 is distributed in different ways. All the flames and incandescents 

 give continuous spectra which die out for even the highest tempera- 

 ture -of these sources at about wave length 300 ju^- Sources giving 

 discontinuous spectra generally extend below this limit of wave 

 length, but often with very feeble radiation in this region. Such, for 

 instance, is the case with the carbon arcs, which show chiefly metallic 



