EFFECTS OF RADIANT ENERGY ON THE EYE. 773 



After 55 years of age, only one out of 12 individuals could see rays 

 below 395 fxjj., that is, in the ultra violet region. The only medium 

 which changes at that age is the lens, so that it is still more definitely 

 proven that the lens establishes the lower limit of spectral vision in 

 the human eye. 



Birch-Hirschfeld ^^ compared the visual threshold or distinguishing 

 power, for various intensities of the same wave length, in the ultra 

 violet region, of the aphakic eye and the eyes of individuals ranging 

 from 14 to 70 years of age. He was thus determining not the ultra 

 violet limit of vision, but the intensity at which a definite wave length 

 which both groups of eyes could see, would become visible. He found 

 that the threshold of the lensless eye exceeded that of the normal eye 

 not inconsiderably, except in the case of a red blind physician who had 

 developed a power of distinguishing small intensity changes that 

 almost equalled that for the lensless eye. As the wave lengths of the 

 light used were diminished, he found that the lensless eye gradually 

 gained more advantage until near 381 /x/x and below it showed far 

 greater sensitiveness than the normal eye. His results thus agreed 

 with those of Widmark, though the same accuracy was not attempted, 

 — (the screen method was used) since his point was only to show the 

 relatively greater sensitiveness of the lensless eye, in the short wave 

 than in the long wave length ultra violet regions of the spectrum. 



The appearance of the ultra violet spectrum has been variously 

 described. Helmholtz characterized it as deep indigo blue under 

 weak illumination to silvery blue under stronger illumination. Seku- 

 lic ^^* and Sauer ^°^ called it silver gray in color. "Widmark's aphakic 

 patients described the first part, 340 nn to 370 fifx, as blue or violet and 

 below that all described it as a weak light gray. More recently 

 Schanz and Stockhausen ^^^, Birch-Hirschfeld^^, and others agree 

 on lavender-gray as the best descriptive term. 



The question as to how this sensation is produced, whether by direct 

 stimulation of the retina or by the intermediation of the phenomenon 

 of fluorescence of the lens or retina, has been variously answered. 

 Soret^*^ favored the latter view, but the work of Widmark*^^ and 

 others shows that when the lens, which fluoresces more than any other 

 part of the eye, is removed, still greater range of vision in the ultra 

 violet region is obtained. Further as pointed out by Widmark*^® 

 and Mascart^*° the ultra violet region appears in sharp lines and 

 bundles, not as a blur of light impossible to focus such as would come 

 from the fluorescing lens. Nor could the fluorescence of the retina 

 make these rays visible and still give a sharp image. Thus according 



