vision of every person would be defective, except in reference to
objects at certain definite distances from the eye.

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928. What modifies the refracting power of lenses? What is necessary
to adapt the eye to view objects at different distances? 929. Where is
the refraction of the rays of light mainly effected? 930. When we view
objects at a distance, what kind of lens is required? Has the eye the
power of adapting itself to different distances? How is it effected?

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_Observation._ It is well known that a separate image is formed on
each eye, and, if they are not in the same direction, the objects will
appear double. This is proved by pressing one eye, so that the rays of
light cannot enter it in the same direction as they do in the other;
consequently, the vision is double.

931. By the action of the muscles of the eye, it is turned in
different directions, so that objects can be examined upon each side,
as well as in front, without turning the body. By the slight or
intense action of the straight muscles, the eye is more or less
compressed, and the form of the globe is changed, together with the
relative positions of the different humors. This modification also
adapts the eye to view objects at different distances.

[Illustration: Fig. 143. 1, A pen, an inverted image of which is painted
on the retina of the eye, at 2. The image of all objects upon the
expansion of the optic nerve, is inverted by the crossing of the rays of
light from objects as they traverse the pupil.]

_Observation._ If the eye is fixed for a time on some object which is
distinguished with difficulty, there is a painful sensation, similar
to that experienced by other muscles of the body when used too long.
This is called "straining the eye."

932. When the refraction of the rays of light is too great, as in
over-convexity of the cornea, or the crystalline lens, or the
vitreous humor, or all of them, the image is formed a little in
front of the retina. Persons thus affected cannot see distinctly,
except at a very short distance. This infirmity is called _near_, or
_short-sightedness_. This defect is in a great measure obviated by
the use of concave glasses, which scatter the luminous rays, and thus
counterbalance the too strong refracting force of the eye.

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What does fig. 143 represent? 931. Why can we see objects at the side
as well as in front of the ey