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

  • Kühne, W. (Willy), 1837-1900.
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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    under green glass. In frogs which had been kept in the dark I found this kind of rod also inconstant. As the outer limbs of the cones in the retina of the frog are very short and come to a fine point, it is possible that a purple colouration, even if present, might not be recog- nized. I have sought evidence of this in the retinas of birds (pigeons and fowls), which possess a large number of cones ; although here I had from the outset to expect difficulties of observation on account of the well-known pigment-globules, and other colourations of the inner limb of the cones. The result of my numerous observations was negative : generally there was no visual purple at all to be recognized in the retinas of these birds. It was impossible for my sense of colour, as well as that of all other persons before whom I laid the fresh retina, to perceive any change of colour in diffuse day or direct sunlight—either in the central portion which in daylight ap- pears to be chiefly coloured by red pigment-globules or in the peripheral greenish-yellow coloured portions. Since both regions contain rods as well as cones, and since at the periphery most elements are free from pigment-globules, I suppose one ought to see the visual purple of birds if it exists. Where in fresh prepara- tions heaps of separated outer limbs lay in thick layers together, I have never seen any indication of purple. Therefore pigeons and fowls have no coloured substance in the outer limb which can be changed by light; and this proves that sight does not entirely depend upon the presence of such a substance. From M. Schultze's discoveries upon the eyes of nocturnal birds, we know that intensely coloured pigment-globules are not present in the retinas of all birds ; and if doubts are found cast here and there upon this important statement of the morphology of the retina, this is most probably due to the modest statement of the great histologist, who does not pronounce decidedly on this point. The doubts of many observers may be due to the fact that they are unable to reconcile the observation of the absence of red globules in the owl's eyes, the globules in this animal being at the most pale yellow, with the mention of a strongly marked red colour in the rods, an appearance which previous to the discovery of visual purple had not been understood. At the