OBSERVATIONS ON SOUNDNESS. 
337 
points of an object which is luminous, at some distance, and 
fall upon a double convex lens, they are brought to a focus 
upon the other side of it; so that an inverted picture is 
formed upon a screen placed in a proper position to receive it. 
Dr. Carpenter writes :—The eye, in its most perfect form— 
such as it possesses in man and the higher animals—is an 
optical instrument of wonderful completeness, designed to 
form an exact picture of surrounding objects upon the retina 
or expanded surface of the optic nerve, by wdiich the im¬ 
pression is conveyed to the brain. The rays of light, which 
diverge from the several points of any object, and fall upon 
the front of the cornea, are refracted by its convex surface 
whilst passing through it into the eye, and are made to con¬ 
verge slightly. They are brought more closely together by 
the crystalline lens, which they reach after passing through 
the pupil; and its refracting influence, together with that 
produced by the vitreous humour, is such as to cause the 
rays that issue from each point to meet in a focus on the 
retina. In this manner a complete inverted image is shown.” 
Dr. Golding Bird, in his ‘ Manual of Natural Philosophy,’ 
states:—Rays of light, on impinging upon the eye, are 
refracted through the transparent cornea, those incident on 
the sclerotic being reflected or absorbed. The cornea may be 
regarded as constituting the anterior surface of a meniscus 
lens, of which the posterior surface is formed by the anterior 
capsule of the crystalline lens, the aqueous humour forming 
the refracting medium of this fluid refractor. The rays of 
light which thus tend to be refracted to a focus, pass through 
the pupillary opening of the iris, those passing too near the 
margin of the lens formed by the anterior chamber being 
reflected or absorbed ; the iris, answering the purpose of the 
perforated diaphragms in microscopes and telescopes, and 
being capable of varying its aperture, possesses advantages 
, altogether unattainable in metallic diaphragms. The pencil 
of rays having passed through the fluid meniscus, impinges 
on the crystalline lens, and is there considerably refracted; 
this refraction is modified by the action of the vitreous 
humour, the last medium into which the pencil passes; and 
finally an inverted image of the object, from the several 
points of which the rays of light are propagated, is painted 
on the retina. All rays which are reflected in the interior of 
the eye, or pass too obliquely for distinct vision, are absorbed 
by the black pigment with which the interstices and folds of 
the choroid coat are imbued.” Sir David Brewster, in his 
^ Optics,’ writes ;—I have found the following to be the 
refractive powers of the different humours of the eye, the ray 
