(Hypermetropia).—If the arrange ments necessary to secure distinct vision when a person looks at objects at varying distances from the eye be understood, the defects of the eye, termed long-sight and short-sight, will be readily comprehended. We have seen that in ordinary conditions of the eye, rays of light from distant objects form a picture on the retina without any effort on the part of the eye. Now suppose the distance between the back wall of the eye and the front is less than usual, other things being usual, rays of light from far-off bodies will reach their focus not on the retina, as they ought to, but behind it, because the retina is not so far back as it ought to be. (Refer to Fig. 181.) If the difference from the normal be slight, the person is able to correct it by a slight effort of accommodation. By this slight effort the lens becomes more convex, brings the rays sooner to a focus, and thus brings the picture forwards so as to make it fall on the re tina,when the object is distinctly seen. The effort required may be so slight that for a long time the person is unaware of it. But the meaning of the condition is, that even when the person is looking at far-distant things, which in ordi nary circumstances he should see distinctly without any movement of the lens, even when looking at these far-off things his eyes are not at rest, but require to focus to make the image fall on the retina. As the object conies nearer the amount of focussing required becomes greater, and the power of accommodating the eye to see distant objects, having begun sooner than is usual, is sooner exhausted. That is, the eye becomes unable to focus any further before the object has come so near the eye as is usual. Thus a person with long-sight is unable to read a letter or newspaper, let us say, when it is held the ordinary distance from the eye, because his focussing power has failed sooner than is customary. He, therefore, holds the paper or letter farther off from his eye than ordinarily is done. The "near point" is farther away from the eye than in ordinary sight. The defect is called "long-sight" on this account. The remedy is evident. Suppose it is at twelve inches from the eye that the power of the lens to become more convex fails, by placing in front of the eye a spectacle whose surface is convex—rounded—the lens is aided, the focus of the rays is brought forward, and the person can now hold his letter or paper nearer and yet see distinctly. (Refer to Section XXIII.) The accommodation of the eyes of a long sighted person is never at rest. The result is that in time a feeling of strain and soreness is produced, and the eyes become red and watery, especially when the person reads, writes, sews, or performs any fine work, since, the nearer the object is, the more effort is required to see it distinctly.
(Myopia) is the opposite condi tion to the former. The distance between the back and front of the eye is greater than usual. When the person looks at distant objects, the focus does not fall on the retina as in ordinary sight, nor yet behind the retina as in long-sight, where the distance is less than usual, but it falls in frontof the retina. (Fig.181, c.) Itis plain that the person can do nothing to correct this. His eyes are at rest, and yet the focus is in front of the retina. Any effort of accommodation would make things worse, by making the lens more convex, and bringing the image still farther forwards. If the lens could be flattened so that the rays were not brought to- a focus so soon, distinct vision would result, but there are no arrangements for doing this. The eyes are at
rest, and in this state the lens has its least degree of forward curve. Now as the object looked at comes nearer and nearer the focus gradually passes back till at length it falls on the retina, and the person then sees the object distinctly. Thus a short-sighted person cannot see persons or things distinctly at a distance. Moreover, a short-sighted person sees distinctly, and without any accommodation by his eyes, that is, his eyes being at rest, an object at the distance for which a person with ordinary sight requires to focus strongly. That is to say, the short-sighted person does not require to bring into play the arrangements for accommo dation so soon as the person with ordinary sight, and thus the accommodation of the person with ordinary sight is exhausted before that of the short-sighted person. Thus, when the ordinary individual has brought printed matter so near his eyes that if he holds it any nearer it is no longer distinct, the short-sighted person can bring it much nearer and still see it distinctly. Indeed to see it distinctly he requires to hold it nearer than the ordinary reading distance. On this account the defect is called "short-sight." It is thus evident that the short-sighted person cannot by any means see things at a distance distinctly, because the picture does not fall on the back wall of the eyeball ; on the other hand he sees things very much nearer than usual. To correct this some arrangement is required which will prevent the rays of light coming so soon to a focus, by which means the image will be produced further back and made to fall on the retina. A concave spectacle—one hollow on the surface--does this, for it slightly disperses the rays, and they are brought to a point later than they would otherwise be. (Refer to Section XXIII.) The Movements of the eye ball is controlled by a set of six small muscles, which, with one exception, are attached to the back part of the cavity in which the eye rests. The muscles pass forwards and are connected by thin flat tendons to the outer coat of the ball, a short distance behind the clear part of the eye—the cornea. Four of these muscles run a straight course, and are called recti cles (Latin rectus, straight). One is attached in the middle line above, another below, and one to each side of the eyeball. They are, therefore, called superior, inferior, internal and external. Acting alone one would turn the eye upwards, another downwards, the third wards, the last outwards. The other two cles bend in their course, and are called oblique muscles. One arises behind in common with the four straight muscles, and passes to the front towards the inner angle of the socket, there it ends in a round tendon and passes over a tendinous pulley. From the pulley it changes its course, proceeds over the eyeball slightly backwards and becomes attached to the ball at its outer side. When it contracts, acting round the pulley, it rolls the ball. Since it proceeds over the eye it is called the superior oblique muscle. The other oblique muscle is inferior oblique. It springs from the lower part of the inner angle of the socket and passes below the ball towards its outer side where it is attached. When it contracts it also rolls the eyeball, but in an opposite direction to the superior oblique. These oblique muscles do not act alone, but in association with one or other of the straight muscles. In combination they duce the varied movements which the eyeball can so freely perform.