Reading-matter should not be held nearer to the eyes than is necessary to make the print appear perfectly sharp and distinct.  No print should be read continuously that cannot be seen clearly at about eighteen inches.  Those who read music are especially liable to strain the eyes, because exact vision is required to follow the notes.  Persons who wear glasses for reading should be careful to use them while reading music, and good light is necessary to avoid any undue strain.

After reading steadily for some time, the eyes should be rested by closing them a short period or by looking at some distant object, even if only for a few moments.  The book, the sewing, and work generally, should be held as far from the eyes as is compatible with good vision.  The natural tendency is to reverse this rule.  We should never read, write, sew, stitch, or otherwise use the eyes when they smart or tingle, or when the sight is dim or blurred.  The eyes are then tired and need a rest.  Much injury may be done by reading in twilight, or by artificial light in the early morning, and by reading and working in badly lighted and ill-ventilated rooms.

Good artificial light is much to be preferred to insufficient sunlight.  The artificial light should be sufficiently bright and steady; a fickering light is always bad.  Riding against a strong wind, especially on a bicycle, may prove hurtful, at least for eyes that are inclined to any kind of inflammation.  The light reflected from snow is a common source of injury to the eyes.  It is a wise caution in passing from a dark room to avoid looking immediately at the sun, an incandescent light, the glistening snow, or other bright objects.

The eyes should never be rubbed, or the fingers thrust into them,[46] and much less when they are irritated by any foreign substance.  The sooner the offending substance is removed the better.

[Illustration:  Fig. 137.—­Lacrymal Canals, Lacrymal Sac, and Nasal ducts, opened by their Anterior Portion.]

340.  Effect of Alcohol upon the Eye.  The earlier and slighter forms of injury done to the eye by the use of intoxicants are quite familiar:  the watery condition of the eye and of the lids, and the red and bleared aspect of the organ.  Both are the result of chronic inflammation, which crowds the blood into the vessels of the cornea, making them bloodshot and visible.  The nerves controlling the circulation of the eye are partially paralyzed, and thus the relaxed vessels become distended.

But more serious results ensue.  Long use of intoxicants produces diseases of the retina, involving in many cases marked diminution of acuteness as well as quickness of vision, and at times distorted images upon the surface of the retina.  In other instances, the congestion of the optic nerve is so serious as to involve a progressive wasting of that organ, producing at first a hazy dimness of vision which gradually becomes worse and worse, till total blindness may ensue.