Volume 5

Treasury of human inheritance / edited by Karl Pearson, R.A. Fisher, L.S. Penrose.

Date:
1909-
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Treasury of human inheritance / edited by Karl Pearson, R.A. Fisher, L.S. Penrose. Source: Wellcome Collection.

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    absorptive properties we must admit that the evidence we have of normal colour vision in the case of many albinos cannot be held to definitely disprove the proposition that the retinal pigment plays some fundamental part in the mechanism of colour vision. We may add in this connection that quantity of pigment, as measured by fairness and darkness of individuals or by light and dark races, appears to have no association with colour sensitivity. Thus we can find no convincing evidence that the pigment of the epithelium is or is not of fundamental importance in the mechanism of colour vision; the pigment of this structure is sensitive to light, but the changes in it induced by light are extremely slow. Evidently we are not yet acquainted with the real significance of this epithelium, which may well lie in some property other than that of its pigmentary content. The position is this—absence of pigment leads to albinism, a condition associated with serious dis abilities of vision which used to be attributed to the lack of pigment, even though precisely the same disabilities of vision had been repeatedly noted in cases of Total Colour-Blindness where pigmentation was believed to be normal; we now know that the defective vision of albinism is probably not due to lack of pigment at all, and we have no reason to believe that the presence of pigment is necessary to vision. In Retinitis Pigmentosa, a disease in which there is marked degeneration of the pigment epi thelium and of the rods and cones, starting in the periphery of the retina and slowly progressing towards the centre, we get an associated night-blindness at a very early stage of the disease: it has been suggested from this that the defect» of the pigment epithelium possibly interferes with the formation of visual purple in the rods and is thus the source of the night-blindness, yet there is no night-blindness in albinism when pigment is absent, and moreover there may be no colour-blindness in retinitis pig mentosa when the epithelium has degenerated. It would appear that the dependence of the formation of visual purple upon the pigment epithelium is due to some other property than that of the presence of pigment, and the integrity of the epithelium does not appear to be necessary to colour vision. Remembering that light can only produce effect when it is absorbed, the obvious suggestion that follows is that perhaps the visual purple may be the factor concerned which is able to discriminate between different wave lengths, and set up some corresponding impulse which ultimately leads to the sensation of colour. Visual purple is a photo-chemical substance found charac teristically in the outer limbs of the rods of the retina, certainly of most vertebrates and probably of all, possibly also in invertebrates, but there has been a considerable difference in the conclusions of various investigators on this point; it has certainly never been demonstrated to be present in cones, and if it is present in the rod-like cones of the fovea in man, or in the case of those creatures whose retinae are said to be free from rods altogether, it can only be in very minute quantities; it is found in certain nocturnal animals, e.g. the owl, in large quantities and in other nocturnal animals, e.g. some bats; its presence has been denied by some authorities, and has been demonstrated by others to occur in very minute traces only. For a full summary of the discoveries concerning visual purple, with bibliographies, we must refer the reader to Hess 1 and Parsons 2 and must confine ourselves here to a brief statement 1 Bill. No. 354, pp. 733-8. 2 Bill. No. 371, pp. 12-15.