ANIMALS. 215 



greatest quantity of the nerves, are the first in forming. Thus the 

 orain and the spinal marrow are the first seen begun in the embryo ; 

 ind, in general, it may be said, that wherever the nerves go, or send 

 their branches in great numbers, there the parts are soonest begun, 

 ind the most completely finished. 



If we examine the eyes of a child some hours, or even some days 

 after its birth, it will be easily discerned that it, as yet, makes no use 

 of them. The humours of the organ not having acquired a sufficient 

 consistence, the rays of light strike but confused-ly upon the retina, or 

 expansion of the nerves at the back of the eye. It is not till about a 

 month after they are born, that children fix them upon objects; for, 

 before that time, they turn them indiscriminately every where, with- 

 out appearing to be affected by any. At six or seven weeks old, they 

 plainly discover a choice in the objects of their attention; they fix 

 their eyes upon the most brilliant colours, and seem peculiarly de- 

 sirous of turning them towards the light. Hitherto, however, they 

 only seem to fortify the organ for seeing distinctly ; but they have 

 still many illusions to correct. 



The first great error in vision is, that the eye inverts every object: 

 and it in reality appears to the child, until the touch has served to 

 undeceive it, turned upside down. A second error in vision is, that 

 every object appears double. The same object forms itself distinctly 

 upon each eye, and is consequently seen twice. This error, also, can 

 only be corrected by the touch ; and although, in reality, every ob- 

 ject we see appears inverted and double, yet the judgment and habit 

 have so often corrected the sense, that we no longer submit to its im- 

 position, but see every object in its just position, the very instant it 

 appears. Were we, therefore, deprived of feeling, our eyes would 

 not only misrepresent the situation, but also the number of all things 

 around us. 



To convince us that we see objects inverted, we have only to ob- 

 serve the manner in which images are represented, coming through a 

 small hole, in a darkened room. If such a small hole be made in a 

 dark room, so that no light can come in, but through it, all the objects 

 without will be painted on the wall behind, but in an inverted posi- 

 tion, their heads downwards. For as all the rays which pass from the 

 different parts of the object without, cannot enter the hole in the 

 same extent which they had in leaving the object ; since, if so, they 

 would require the aperture to be as large as the object ; and, as each 

 part, and every point of the object, sends forth the image of itself on 

 every side, and the rays, which form these images, pass from all points 

 of the object as from so many centres, so such only can pass through 

 the small aperture as come in opposite directions. Thus the little 

 aperture becomes a centre for the entire object ; through which the 

 rays from the upper parts, as well as from the lower parts of it, pass 

 in converging directions ; and, consequently, they must cross each 

 other in the central point, and thus paint the objects behind, upon thu 

 wall, in an inverted position. 



It is in like manner easy to conceive, that we see all objects double, 

 whatever our present sensations may seem to tell us to the contrary 

 For, to convince us of this, we have only to compare the situation of 



