206 MANUAL OF JIODEIIN FARRIER V. 



two globes, the half of the one, a, plate x. fig. 8, smaller, 

 and transparent in front ; and of the other, B, which is 

 larger, with an opaque coat behind. 



It is an established law in optics, that all objects becoue 

 visible from the rajs of light which flow from these objects 

 into the eye. These rays pass through the pupil and fall 

 upon the retina, which is a fine expansion of the optic 

 nerve, interwoven like net- work in the back part or bottom 

 of the eye, and there the rays form a picture of the object, 

 whose apparent bulk depends upon the size of such picture 

 so formed upon the retina. We shall suppose the animal 

 looking at an arrow with the barb of it downwards, c, d. 

 From every part of the arrow rays of light will be sent 

 forth in straight lines, and in passing through the pupil, 

 plate iii. fig. 5, hi it is clear that those which flow from the 

 under portion of the object, c, must flow upwards, while 

 those above, d, must pass downwards ; and pursuing this 

 principle, all the intermediate rays,/, will intervene, con- 

 sequently a reversed picture of the object will be formed 

 upon the retina, as seen at g, h. 



Paley makes the following interesting observation on this 

 subject : he says, ■' In considering vision as achieved by 

 means of an image formed at the bottom of the eye, we can 

 never reflect without wonder on the smalluess, yet correct- 

 ness of the picture, the subtlety of the touch, and the fine- 

 ness of the lines. A landscape of five or six square leagues 

 is brought into a space of half-an-inch in diameter ; yet 

 the magnitude of objects which it contains are all preserved, 

 are all discriminated in their magnitudes, positions, figures, 

 and colours. A stage-coach passing at its ordinary speed 

 for several minutes, passes in the eye only over one twelfth 

 of an inch, yet is the change of place in the image distinctly 

 perceived throughout its whole progress." 



