light reach the back of it before being focussed; it is therefore clear that such an eye must be continually using its accommodative power in order

to see at all. It is to be remembered that an eye of correct form does not need to call into use the

muscles controlling its extra focussing power when regarding distant objects (see Fig. B), but only when regarding near objects; as in reading, writing,

and like occupations, as shown in Fig. D, where the lens is noticeably more full or convex.

As an over-sighted eye must continually exert itself at all distances to focus rays of light, it is almost certain to overuse its accommodative power; and therefore over-sight is a very usual cause of weak sight. An eye correctly formed may become weak-sighted; but an eye that is too flat is, if put to much near use, almost sure to get fatigued, and

in time to develop all the unpleasant features of this affection. Having to use its accommodative power for distant as well as for near objects, there is no rest for it except during the hours of sleep.

CONVEX GLASSES FOR OVER-SIGHT.

The remedy for over-sight suggests itself at once. If the eye is too flat, of course a convex glass

makes it less so, practically, and thus removes the effect of its defective form. Convex glasses in the form of eye-glasses or spectacles are therefore imperative to prevent the development of weak sight as well as to aid vision. What the strength of the glasses should be will depend upon the degree of faulty formation of the eye-ball. An oculist is the proper person to determine this. My object here is to make the fact clear that no method of resting the eyes, and no kinds of rules and regulations, can probably prevent such eyes becoming weak-sighted, painful, and useless, except convex glasses be worn; and that these will, to a great extent, render such eyes serviceable and comfortable. Those who are over-sighted, are, however, advised to use their eyes with discretion, and to regard the rules and regulations given on pages

14-17.

NEAR-SIGHT (MYOPIA).

This is defective sight of a nature exactly the opposite of that of over-sight. The eye is too full, or too deep from front to rear, so that the rays of light are brought to a focus before reaching the retina, at the back of the eye (see Fig. F). Vision is, therefore, indistinct and hazy unless objects are brought very near. It will be remembered that, in order to see clearly, rays of light must be focussed

exactly on the retina; but, in a near-sighted eye, the distance between the retina at the back of the eye and the front of the eye-ball is so great, that the convex form of the lens brings the rays of light to a point, in spite of all effort to the contrary, before they reach the retina. The exercise of the accommodative or adjusting power of the eye is of

no use, or even worse than useless, because, when brought into service, its function is to make the lens more convex (see p. 12); and this addition to the refractive power of the eye would, of course, focus the light still sooner, and so increase rather than help the difficulty.

THE USE OF CONCAVE GLASSES.

To render vision clear in near-sight, it is of course necessary to resort to some means by which the rays of light may be prevented from being

focussed too quickly after entering the pupil. That is, we wish to do exactly the opposite of that which we wish to do in over-sight, at page 28.

This is accomplished by putting before the eyes concave glasses) either in the form of eye-glasses or spectacles. A concave glass is one which is hollowed out, or thinner at the centre than at the edges. A convex glass, suitable for far-sight or over-sight, is one thicker at the centre than at the edges. Such glasses help to focus the rays of light; while a concave glass, by diminishing the effect of the convex form of the eye-ball, hinders the focussing of the rays of light, and thus permits them to reach the back of the eye in proper form for good vision.

NEAR-SIGHTED EYES ARE GENERALLY DISEASED.

Near-sight is usually accompanied by a disease in the back part of the eye. This is almost always the case in the near-sight of children and youth. Besides this disease in the back part of the eye, such eyes are often weak-sighted, -the weak sight being mostly due to weakness and fatigue of the muscles of convergence (M C, Fig. A, p. 9). The muscles of convergence are very liable to be overworked, because print in reading and fine objects of all kinds must be held quite near, which necessi