The photosensitivity of visual purple solutions and the scotopic sensitivity of the eye in the ultra-violet

Abstract

Previous measurements of the spectral variation of the photosensitivity of visual purple solutions have been extended into the far ultra-violet. The products have been found to be the same as in the visible. Dark adaptation curves, produced by various observers, including one with an aphakic eye, were of the same type at 365 as at $546$ $\text{m}\mu $. The limiting scotopic sensitivity (the reciprocal of the potential retinal illumination in quanta/sec./sq. mm. which is just sufficient to excite vision) of normal eyes at $365$ $\text{m}\mu $ is more than ten thousand times smaller than that to be expected from the photosensitivity at this wave-length, a fact which is attributed to the presence of light absorbing substances in front of the retina. The scotopic sensitivity of the aphakic eye was, however, as expected, indicating the absence of absorbing substances. Indeed the aphakic eye was practically as sensitive at 365 as it was at $546$ $\text{m}\mu $. The limit of vision in the ultra-violet was accurately determined and, for normal eyes, found to correspond to the threshold of absorption of the lens, $309$ $\text{m}\mu $. The limit for the aphakic eye (no lens) corresponded to the threshold of absorption of the cornea, $298$ $\text{m}\mu $.