Contributions to the comparative anatomy of the mammalian eye : chiefly based on ophthalmic examination / by George Lindsay Johnson.

Johnson, G. Lindsay (George Lindsay), 1853-1943.

Date:: [1900]

Credit: Contributions to the comparative anatomy of the mammalian eye : chiefly based on ophthalmic examination / by George Lindsay Johnson. 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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$tion, and their proximity to each other, and to the diaphragm. It is evident that the increased size of the cornea will allow rays of much greater obliquity to be refracted into the eye, and a spherical lens will diminish the distortion produced by the focus of those oblique rays, and that all the more, since the image is received upon a surface nearly equidistant from the nodal points. In our eye, possessing only a small angle of acute vision, a blurred image produced at a point near the periphery is of no conse- quence, but in the case of those animals which have a widely extended sensitive area and need to see all round them, a sharp focus for peripheral rays is of much greater importance. IX. Physiological Considerations. It is obviously beyond the scope of this paper to attempt any theory of vision or of colour perception, or even to see to what extent the facts which we have recorded can be brought into harmony with existing views. It will not be possible to do more than call attention, and that only in a very cursory manner, to some few among many points which have specially impressed me in the course of this work. In referring to a number of our text-books I have been struck with the difference between the explanation given for the ophthalmoscopic appearance and the descrip- tions of the eye which underlie the theories of vision. In the first instance we are told that the ophthalmoscopic image is due to reflection from the choroid and its vessels, whilst on the other hand we are told that the impinging light is absorbed by the black opaque epithelial layer of the retina. Now it seems to me that reflections from the choroid can only be seen if the epithelial layer is transparent, or at least trans- lucent ; and if this is so, the light is not absorbed by the epithelial layer, but we see by reflected and not by incident light, unless it be that both play a part in vision. In the last edition of Helmholtz's ' Physiologische Optik ' the view is expressed that we see both by incident and reflected light; and, according to Bruoke, the reflection has the effect, that the light which has struck the sensitive elements of the retina by impinging on them, strikes and affects them again on its return, at least in those eyes, he says, which possess a Tapetum lucidum. Now we have seen that in eyes which have no tapetum, the membrane of Bruch and the internal layers of the choroid are transparent, and that the epithelial layer is very translucent. We have seen that in eyes which have a Tapetum cellulosum, the inner layers of the choroid are opaque and colourless, the colour being exclusively due to the translucent epithelial pigment crystals. Lastly, we have found that the Tapetum fibrosum consists of certain layers within the choroid which are structurally coloured, the colour being to some extent modified by translucent coloured epithelial crystals. I venture to think that these facts must be taken into consideration in forming any theory of vision or colour perception. Having had occasion to study the work of Lippmann on colour photography, I could not help being struck by the similarity of his device and the conditions which vol. cxciv.—b. K$