A manual of physiology : with practical exercises / by G.N. Stewart.

  • Stewart, G. N. (George Neil), 1860-1930.
Date:
1900
Catalogue details

Licence: In copyright

Credit: A manual of physiology : with practical exercises / by G.N. Stewart. Source: Wellcome Collection.

Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.

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    images and impression of a plane surface on the one hand, and differ- ence of retinal images and impression of solidity on the other, is proved by the facts of stereoscopy. It is evident that if an exact picture of the solid object as it is seen by each eye can be thrown on the retina, the impression produced will be the same, whether these images are really formed by the object or not. Now, two such pictures can be produced with a near approach to accuracy by photographing the object from the point of view of each eye. It only remains to cast the image of each picture on the corresponding retina, while the eyes are converged to the same extent as would be the case if they were viewing the actual object. This is accom- plished by means of a stereoscope (Fig. 306). It is found that the resultant impression is that of the solid object. It is impossible to reconcile this with the doctrine of strictly identical points. A pair of identical pictures gives with the stereoscope not the impression of a solid, but of a plane surface. If the relative position of any two points differs in the two pictures, the blended picture has a corresponding point in relief. So great is the delicacy of this test that a good and a bad banknote will not blend under the stereoscope to a flat surface, and the method may be actually used for the detection of forgery. When the pictures are interchanged in the stereoscope so that the image which ought to be formed on the right retina falls on the left, and that which is intended for the left eye falls on the right, what were projections before become hollows, and what were hollows stand out in relief. The pseudoscope of Wheatstone is an arrangement by which each eye sees an object by reflection, so that the images which would be formed on the two retinae, if the object were looked at directly, are interchanged, with the same reversal of our judgments of relief. Visual Judgments.—We say judgments of relief; for what we call seeing is essentially an act that involves intellectual processes. As the retina is anatomically and developmentally a projection of the brain pushed out to catch the waves of light which beat in upon the organism from every side, so physiologically retina, optic nerve and visual nervous centre are bound together in an indissoluble chain. We cannot say that the retina sees, we cannot say that the optic nerve sees—the optic nerve in itself is blind—we cannot say that the visual centre sees. The ethereal waves falling on the retina set up impulses in it which ascend the optic nerve; certain portions of the brain are stirred to action, and the resulting sensations of light springing up, we know not where, are elaborated, we know not how (by processes of which we have not the faintest guess), into the per- ception of what we call external objects—trees, houses, men, parts of our own bodies, and into judgments of the relations of these things among themselves, of their distance and movements. A child learns to see, as it learns to. speak, by a process, often un- conscious or subconscious, of ' putting two and two together.' The musical sounds united and terminated by noises which make up the spoken word ' apple' are gradually associated in its mind with the