ro 4 o THE SENSES 



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 retina?, if the object were looked at directly, are inter- 

 changed, 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 perception 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 

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

 smooth and round object, and the gustatory and olfactory sensations 

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

 perience 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 movements, 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 agree- 

 able to the youthful palate will follow. At length the child comes to 

 believe, and, unless he happens to be specially instructed, carries his 

 belief with him to his grave, that when he looks at an apple he sees a 

 round, smooth, tolerably hard body, of definite size and colour; while 

 in reality all that the sense of sight can inform him of is the difference 

 in the intensity and colour of the light falling on his retina when he 

 turns his head in a particular direction. 



An interesting illustration of the role of experience in shaping 

 our visual judgments is found in the sensations of persons born 

 blind and relieved in after-life by operation. A boy between 

 thirteen and fourteen years of age, operated on by Cheselden, 



