THE SENSES 807 



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 



