the different lenses. 1, A plane
lens. 2, A globe lens. 3, A convexo-convex lens. 4, A plano-convex lens.
5, A concavo-concave lens. 6, A plano-concave lens. 7, Meniscus. 8, A
concavo-convex lens.]

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925. Have the cornea and the humors of the eye different degrees of
density? What is said of the crystalline lens? What effect has the
different density of the parts of the eye upon the light admitted to
this organ? 926. What kind of lenses do the humors exhibit? 927. What
modifies the refracting powers of transparent mediums? How does this
principle apply to the humors of the eye?

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927. The different degrees of convexity or concavity also modify the
refracting character of transparent mediums. The crystalline lens is
of different degrees of convexity on its two sides. The convex
surfaces of the aqueous and vitreous humors are segments of circles,
of different diameters from their concave surfaces. (Fig. 139.) All
these circumstances still further influence the refracting character
of the visual organ. The achromatic arrangement of the transparent
refracting mediums of the eye, remedies the aberration of refraction
in the different portions of the eye.

928. Again, the refracting power of lenses is modified by their
convexity or concavity. The more convex a lens is, the shorter the
distance from the refracting medium, where the different refracted
rays converge to a focus. To adapt the eye to view objects at
different distances, requires a change in the refracting power of some
of the transparent mediums of the eye.

929. Both surfaces of the crystalline lens are oval, not spherical,
and the refraction of the rays of light is mainly effected in this
portion of the eye. Change the inclination of this lens, so that
different portions of its anterior surface shall be directly behind
the pupil, and its refracting power is increased or diminished, as the
surface presented is more or less convex.

930. To view objects at a distance, a less convex lens is needed than
in examining articles very near the eye; and this organ, from its
structure, has the power of adaptation to different distances. It is
supposed that the muscular substance of the ciliary body and processes
changes, by its contraction, the inclination of the crystalline lens.
Without this, or some other adapting power, a picture of objects at
different distances would not be formed upon the retina, and the