hich conveys the tears to the nasal cavity on the same side
of the nose. When by evaporation the eyeball becomes too dry, the lids
close reflexively and spread a fresh layer of tears over the surface. Any
excess is passed into the nostrils, where it aids in moistening the air
entering the lungs.

HYGIENE OF THE EYE

*Defects in Focusing.*--The delicacy and complexity of the sense organs of
sight render them liable to a number of imperfections, or defects, the
most frequent and important being those of focusing. Such defects not only
result in the imperfect vision of objects, but they throw an extra strain
upon the nervous system and may render the process of seeing exceedingly
painful.

A normal eye is able, when relaxed, to focus light accurately from objects
which are twenty feet or more away and to accommodate itself to objects as
near as five inches. An eye is said to be _myopic_, or _short-sighted_,
when it is unable to focus light waves from distant objects, but can only
distinguish the objects which are near at hand. In such an eye the ball is
too long for the converging power of the lenses, and the image is formed
in front of the retina (_C_, Fig. 164).

[Fig. 164]

Fig. 164--*Diagrams illustrating long-sightedness and short-sightedness*,
and method of remedying these defects by lenses. _A._ Normal eye. _B._
Long-sighted eye. _C._ Short-sighted eye.

A _long-sighted_, or _hypermetropic_, eye is one which can focus light
from distant objects, but not from near objects. In such an eye the ball
is too short for the converging power of the lenses and the image tends to
form back of the retina (_B_, Fig. 164). These defects in focusing are
remedied by wearing glasses with lenses so shaped as to counteract them.
Short-sightedness is corrected by concave lenses and long-sightedness by
convex lenses, as shown in diagrams above.

_Astigmatism_ is another defect in the focusing power of the eye. In
astigmatism the parts of the eye fail to form the image in the same plane,
so that all portions of the object do not appear equally distinct. Certain
parts of it are indistinct, or blurred. The cause is found in some
difference in curvature of the surfaces of the cornea or crystalline lens.
It is corrected by lenses so ground as to correct the particular defects
present in a given eye.

Whenever defects in focusing are present, particularly in astigmatism,
ex