174 SATURDAY LECTURES. 



lation of this object to itself? This is a question which 

 puzzled philosophers for a. long time, and many explana- 

 tions were given to account for it. By some it was supposed 

 that in some way the image was "righted " before it reached 

 the retina, by others, it was thought that it was turned right 

 side up on its passage from the retina to the brain. But it 

 is by no means necessary to assume any such mysterious 

 processes. The explanation, when we once find it, is sim- 

 ple enough. The image, as we have before remarked, is 

 undoubtedly formed upside down on the retina. But we 

 must bear in mind that this image only produces an im- 

 pression — the interpretation of this impression is made b}'- 

 the brain through the faculty we call the judgment. Now 

 the judgment is often the result of several impressions, re- 

 ceived, it may be, from several different sources. In the 

 case of the rattle, the impression made by the image must 

 be verified by the impression of touch. The infant reaches 

 out its hand and finds that the globular end of the rattle 

 corresponds in direction to its feet or downward, while the 

 ring end corresponds to an upward direction or towards its 

 head. It does not make any difference that the lower or 

 round end of the image corresponds to the upper part of 

 the object, and vice versa. The position of the retinal image 

 is a matter of no importance, so long as the judgment, 

 guided by touch, places the object in its proper relation with 

 the individual and surrounding objects. Physiologists call 

 this placing of objects properly in space the "law of pro- 

 jection," which, being interpreted, means that all impres- 

 sions made on the upper parts of the retina are "projected" 

 01' referred downward, and all impressions made on the 

 lower parts of the retina are projected upward ; and as the 

 relative position of the objects in the external world are in 

 no wise affected by their inverted images on the retina, no 

 confusion can arise. 



The same principle applies when we judge of the distance 

 and size of objects. The size of a retinal image gives us no 

 idea whatever of the size of the object or its distance from 

 us. These two ideas are inseparably connected in the 



