Alleged effects of the near ultraviolet on human vision.

It has been reported that short exposures of the human eye to near ultraviolet radiation in the wavelength range 290–365 mμ produce appreciable changes in subsequent dark adaptation, and a lasting depression of visual sensitivity, rod and cone. These reports are hard to reconcile with the fact that only traces of such radiation reach the retina. Wavelengths shorter than 300 mμ are absorbed by the cornea of the eye, and they can do damage; longer wavelengths up to 400 mμ are absorbed in the lens. The assertion that such radiations affect the visual sensitivity is reexamined in the present paper. Exposures to ultraviolet light identical with those referred to produce no appreciable effects on subsequent dark adaptation of the normal eye, rod or cone. In lensless (aphakic) subjects, near ultraviolet radiation in the neighborhood of 365 mμ does reach the retina and is clearly visible. Even in such persons, exposure to the near ultraviolet does not affect the threshold of rod vision after 20 minutes of dark adaptation. The early portions of dark adaptation—the adaptation of the cones, and sometimes also the early rod thresholds—may be raised; but these changes are caused, not by intrinsic effects of the ultraviolet, but by the increased brightness to which it gives rise in the aphakic eye.It is concluded that ultraviolet radiations which might harm the retina do not reach it; those which reach the retina do it no harm, and can at most be seen.