406 THE PROGRESS OF VETERINARY SCIENCE. 
cessful practice, based on our increased knowledge of patho- 
logy and physiology. The caloptric test for cataract is a 
triumph of the application to art of physiological knowledge 
and philosophical principles. As it may be new to some of 
your readers I may describe it. Take a horse into a dark 
place, bring a light within six inches of one of his eyes. By 
looking steadily for a few minutes into the eye, you will ob- 
serve three reflections of the light used, two of them in a 
vertical and one in an inverted position ; now move the light 
a few inches from right to left, looking steadily into the eye, 
when the two vertical images will move in the direction of 
the light, and the inverted one in the opposite direction. If 
cataract be present no inverted image is visible ; the lens being 
opaque, no rays of light can pass through it. In connection 
with cataract we may allude to its nature and treatment, as 
propounded by its discoverer — the late Sir David Brewster. 
This great philosopher found that, when the lenses of 
several animals, and perhaps of all, are kept in distilled 
water, the rings or quadrants of polarized light change their 
form, and even their colour, as if the lens was assuming a 
new and more complex structure. After a certain time the 
lens bursts from the breaking of the capsule or bag which 
encloses it. The distilled water had been absorbed in such 
quantity as, at first, to stretch and then tear the capsule, and 
the effect of its expansion was to give it a polarizing struc- 
ture, which modified the polarizing structure of the lens 
itself. This experiment goes to show that the disease of 
soft cataract in the human eye may be produced by the lens 
absorbing too much water, from an excess of water in the 
aqueous humour, and that dry cataract may be produced 
by the lens not receiving a proper supply of water, in con- 
sequence of the aqueous humour containing too much 
albumen. In short, that the lens is nourished by imbibition 
from the aqueous humours, and in certain conditions of sys- 
tem the humours get too dense to allow of the passage of 
fluid to the lens, so that it gets dried up ; the serrated mar- 
gins of the fibres composing the lens appear separate from 
each other, producing dry catract, or, vice versa , producing 
soft. He further thought that, by puncturing the cornea 
and allowing the aqueous humours to escape (the chamber 
became filled up with a less dense fluid), or by injecting a 
quantity of water into it, a cure might be effected. By 
taking advantage of this and similar discoveries, our science 
has made good progress within the last twenty years ; the 
theories of the inflammatory process, and the natural course of 
disease have been greatly modified within that period. 
