ORGANS OF SENSE. 173 
upon the surface of the cornea, which being convex and much more dense 
than air, refracts them, and causes them to converge towards a distant 
focus. In passing through the anterior chamber, this convergence is slightly 
counteracted, and the degree of refraction is about equal to that which 
would have occurred had they impinged at once upon the convex surface 
of the aqueous humor, supposing the cornea not to have been interposed. 
Much of the light that traverses this fluid falls upon the iris, which, like the 
stop or diaphragm in any optical instrument, excludes it, and this light is 
reflected back again through the cornea. Hence we see the fibrous texture 
and variegated color of the iris when we look upon the eye of another. 
Those pencils of rays only which pass through the pupil are subservient to 
vision. As these pass through the lens they undergo two refractions, 
because this medium is convex on both surfaces, and is more dense than. 
either the aqueous or vitreous humor. These refractions increase the con- 
vergence of the rays, and bring them to their proper focus upon the sentient 
surface of the retina, on which is formed an accurate image of the various 
external objects from which the rays of light have been received. This 
image is inverted. What is above in the object is below in the image. 
The right side of the object-is to the left, and the left to the right, while the 
relative position of its different parts remains the same. But as all sur- 
rounding objects, if seen, must be equally reversed, the relative position of 
all objects, therefore, remains unchanged, and there can be no comparison 
between what is erect and what is inverted, and, therefore, the sense 
acquires a correct idea of the position of objects. The fact, however, 
of our seeing objects erect, notwithstanding their images are inverted on 
the retina, as also that of single vision, from the simultaneous action of 
both eyes, and many other phenomena, has given rise to some interesting 
inquiries and to much speculation respecting the functions of the retina and 
optic nerves. | 
As the accurate convergence of all these pencils of rays to their respec- 
tive foci upon the retina is necessary for the formation of a perfect image, 
and for clear and distinct division, certain nice adjustments in the appa- 
ratus are required, and have accordingly been supplied, and which in 
artificial optical instruments are attained with difficulty by mechanical 
skill. An image of an object, formed by a simple refracting medium, is 
liable to certain imperfections or aberrations of light. These are princi- 
pally three, and are technically termed, first, aberration from sphericity ; 
second, from parallax; and, third, chromatic aberration. Each of these 
we shall briefly explain, with a view of considering how they are obviated 
in the living eye. 
Aberration from sphericity arises from this cause: a refracting spherical 
surface does not unite the parallel or diverging rays of a pencil exactly 
into one focus, because the lateral rays converge sooner than the central ; 
hence arises a certain degree of confusion, which requires much attention 
to correct in the construction of the microscope or telescope. This correc- 
tion is effected in the eye partly by the iris shutting off all circumferential 
rays, and partly by the form and texture of the lens. Its surfaces are not 
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