926 A MANUAL OP PHYSIOLOGY 



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 accomplished 

 by means of a stereoscope (Fig. 403). 



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 

 geometrical 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 arrange- 

 ment 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 

 unconscious 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 visual sensation of a red or green object, the tactile sensation of 

 a smooth and round object, and the gustatory and olfactory sensa- 

 tions which we call the taste or flavour of an apple. And as it is by 

 experience that the child learns to label this bundle of sensations 

 with a spoken, and afterwards with a written, name, so it is by 

 experience that it learns to group the single sensations together, and 

 to make the induction that if the hand be stretched out to a certain 

 distance and in a certain direction i.e., if various muscular move- 

 ments, also associated with sensations, be made the tactile sensation 

 of grasping a smooth round body will be felt, and that if the further 

 muscular movements involved in conveying it to the mouth be 

 carried out, a sensation agreeable to the youthful palate will follow. 

 At length the child comes to believe, and, unless he happens to be 



