206 
MANUAL OF MODERN FARRIERY. 
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 rajs of light which flow from these objects 
into the eye. These rajs 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 eje, and there the rajs 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 everj part of the arrow rajs of light will be sent 
forth in straight lines, and in passing through the pupil, 
plate iii. fig. 5, h , 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 rajs,/, will intervene, con- 
sequentlj a reversed picture of the object will be formed 
upon the retina, as seen at g, h. 
Palej makes the following interesting observation on this 
subject: he sajs, “ In considering vision as achieved bj 
means of an image formed at the bottom of the eje, we can 
never reflect without wonder on the smallness, jet correct¬ 
ness of the picture, the subtletj 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; jet 
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 ordinarj speed 
for several minutes, passes in the eje onlj over one twelfth 
of an inch, jet is the change of place in the image distinctly 
perceived throughout its whole progress.” 
