On the photochemistry of the retina and on visual purple / translated from the German of W. Kühne ; edited, with notes, by Michael Foster.

  • Kuhne, W, 1851-1930.
Date:
1878
Catalogue details

Licence: Public Domain Mark

Credit: On the photochemistry of the retina and on visual purple / translated from the German of W. Kühne ; edited, with notes, by Michael Foster. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    the bleacliing of the purple, which hitherto had been put for- ward as a suspicion only, must therefore be considered really to exist, and I imagine it will be thought not only not absurd, but even altogether natural. We must suppose it to occur in all cases where our vision-power is either diminished or annulled in consequence of exposure to light, and how easily that may occur is known to every one who is versed in after-images, and the fixation of sight. I might maintain that we cannot fix our sight, without winking, for 30 seconds on a large bright light without becoming incapable of seeing it, and I find nothing wonderful in the fact that the mammalian eye, after a minute's constant exposure to moderate light, is blinded in the spots where in the visual image the light fell on the retina, indeed is so far blinded that the purple either no longer exists there or is visibly decolourized, and has diminished so much, that a con- siderable time is necessary before such regeneration can take place as to render it again visible. In such cases we must suppose the existence of, and indeed on examination shall dis- cover the presence of, an optogram that is an after-image in the proper sense of the word. It may be urged that optography depends on changes in the rods only, not in the purple free cones, and therefore on processes taking place in a visual- apparatus undoubtedly far less perfect than that of the cones, or that which exists in the region of distinct vision; and it might further be urged that the imperfection of our peripheral vision, in spite of the exist- ence of the scattered cones which exist in the anterior region of the retina, is in many respects remarkable, and especially so because the functional activity of the rods is therein involved. Under this view, optographic methods which depend princij)ally on an incompleteness of the regeneration processes might well be expected to indicate the most varied 'acts of vision. It is well known how rapidly objects seen by indirect vision dis- appear when the eye is well fixed; and this would seem to shew, even more strongly than does the persistence of after-images, that the regeneration of the purple in the rods is a slow pro- cess, and has therefore the same significance as the photo- phobia so characteristic of creatures in whose retinas the cones ,