THE EYE AND ITS VISUAL STRUCTURE 1 79 



posed of the parts of 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 

 become visible from the rays 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 down- 

 wards, 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 in, fig. 5, k, it is 

 clear that those which flow from the under portion 

 of the object, c, must flow upwards, while those 

 above, d, must pass downw^ards ; and pursuing this 

 principle, all the intermediate rays, /y will intervene, 

 consequently a reversed picture of the object will be 

 formed upon the retina, as seen at ^, k. 



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 smallness, yet correctness, of the picture, the 

 subtlety of the touch, and the fineness 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 pre- 

 served, are all discriminated in their magnitudes, 

 positions, figures, and colours. A stage-coach pass- 

 ing at its ordinary speed for several minutes, passes 



