516 PHYSIOLOGY OF THE EYE OF THE HORSE. 
trine. These are great names, but the result of their industry 
has placed men, otherwise at an immeasurable distance, nearly 
upon a level with themselves. For my own part, I have read 
these, and all other standard authorities, as much as I could 
with an unprejudiced mind, with a bias or intention neither of 
believing nor of doubting ; and when, in my honest search after 
truth, I could not coincide with their theories, I have adopted 
one from another quarter, or one of my own : therefore, gentle¬ 
men, you will not accuse me of presumption if I differ from au¬ 
thorities, no matter how high. 
I confess that I agree with Dr. Ramsden, who attributes far 
more to the cornea than to the lens; and which he proved to 
be correct by his ingenious experiment of fixing the head, and 
observing the changes in the convexity of the transparent cor¬ 
nea as the eye was directed to near.or to distant objects. 
In considering vision to be achieved by means of an image 
formed at the bottom of the eye, Paley, in his Natural Theology, 
thus remarks:—We never reflect without wonder on the small¬ 
ness, yet correctness of the picture, the subtleness of the touch, 
and fineness of the lines. A landscape of six or eight square 
leagues is brought perfectly into a space of half an inch in 
diameter; and yet the multitude of objects which it contains 
are all preserved, and are all discriminated in their magnitudes, 
positions, figures, colours; and a stage-coach passing at its or¬ 
dinary speed for several minutes, passes in the eye only over the 
one-twelfth of an inch, yet is the change of place in the image 
distinctly perceived throughout its whole progress.’^ 
I shall not attempt any kind of description of the nature, pro¬ 
perties, and laws of light; but it is absolutely and indispensably 
necessary for vision. Light is derived from various sources :— 
Ist, from the sun and stars, the great illuminators of the uni¬ 
versal system ; 2d, from combustion ; and, 3dly, from chemical or 
electrical or galvanic action, besides which, and how far indepen¬ 
dent of these I am not prepared to say, there is a phosphorescent 
light, both natural and artificial. 
Light consists of very minute particles, emitted in straight 
lines in every direction, and of the smallness of which it is im¬ 
possible to form an adequate conception. It travels at a rate of 
200,000 miles in a second of time. It is, after all, a compound 
body, for every ray of light is composed of particles capable of 
reflecting very different colours. We are at little loss to account 
for the reason of the image being depicted upside down upon the 
retina, for the rays rendered convergent by the cornea and the 
lens, and being brought to a focus, and still continuing their 
course, those"which impinge on the upper part of the cornea must 
strike the lower part of the retina, and those which enter from 
