makes a perfect image of objects at all these distances. This is called its range of distinct vision. It has but one limit, viz., the nearer limit of five inches. Now in the passive state of the eye, as for instance in gazing on vacancy, or when the eye is taken out of the socket as a dead instrument, it is prearranged for perfect image of objects at an infinite distance. Its focus of parallel rays in a passive state is on the retina. For all nearer objects, a voluntary effort is necessary to throw the image on the retina, which effort is greater as the object is nearer, until it is limited at the distance of about five inches. The normal eye, therefore, is like a camera, which, when pushed up as much as possible, is arranged for making a perfect image of sun, or moon, or a distant landscape, but can by drawing the tube be adjusted to shorter and shorter distances up to five inches, but not nearer.

The myopic eye, on the other hand, is not prearranged for perfect image of distant objects. Its focus for distant objects (focus of parallel rays) is not on the retina, but in front of it. The refractive power of the lenses in their passive state is too great, or else the receiving screen (retina) may be regarded as too far back from the lens, viz., at S" S", Fig. 7, page 27. The rays have already reached focus, crossed, and again spread out before they reach the retina. An object must be brought much nearer before its perfect image will be thrown on the retina. Within this farther limit of perfect image, however, it has its own range of adjustment, like the normal eye. The range of the normal eye is from infinite distance to five inches. In the myopic eye the range may be from a yard to four inches, or from a foot to three inches, or from six inches to two inches, or even from three inches to one inch,

according to the degree of myopy. The amount of ocular adjustment or change in the lens to effect these ranges is as great as for the normal range from infinite. distance to five inches, but the latter is a far more useful range. The myopic eye, therefore, is like a camera which was never intended to be used for taking distant objects, which, therefore, when shortened to the greatest degree, is still too long in the chamber for distant objects, but is adapted only for near objects within a certain limited range.

It is evident, then, that, the defect of the myopic eye being too great refractive power of the lens in a passive state, this defect may be remedied by the use of concave glasses, with concavity just sufficient to correct the excess of refractive power, and therefore to throw the image of distant objects back to the retinal screen in the passive state of the eye. The eye then adjusts itself to all nearer distances, and becomes in all respects a normal eye. From the nature of the defect (structural defect), it is evident that the glasses must be worn habitually.

Presbyopy, or Old-Sightedness.-This defect is often called long-sightedness, or far-sightedness; but this is a misnomer, based on a misconception of its true nature. It is obviously impossible to have an eye more long-sighted than the normal eye, for this defines with perfect distinctness the most distant objects, such as the moon or the sun when the dazzling effect is prevented by smoked glass. It is usually regarded as a defect the reverse of near-sightedness. As near-sightedness is the result of too great refractive power in a passive condition, so this is supposed to be a too small refractive power in the same condition. As the myopic eye throws the focus of parallel rays in front of the retina,

so it is supposed the presbyopic eye throws the focus of parallel rays behind the retina, because the retina is too near the lens, at S' S', Fig. 7, page 27. It is further supposed that the change which takes place with age is a flattening, and therefore a loss of refractive power, of the lenses of the eye. It is constantly asserted, therefore, that the myopic eye may be expected to become normal with age.

Now this view of the nature of presbyopy is wholly wrong. The presbyopic eye sees distant objects perfectly well, and precisely like the normal eye. Its passive structure is therefore unaltered. It makes a perfect image of distant objects on the retina, like the normal eye. Its focus of parallel rays is on the retina, not behind it. It is therefore normal in its passive state, or in its structure. The defect, therefore, consists not in a change of the structure which originally adapted it to the imaging of distant objects, but in the loss of power to adjust for near objects. And this loss of adjusting power is, again, probably the result of loss of the elasticity of the crystalline lens. In the normal young eye, when the ciliary muscle pulls forward the lens curtain, and thus relaxes its tension, the lens by its elasticity swells and thickens, and becomes more refractive. In the presbyopic eye, the ciliary muscle pulls, and the curtain or capsule relaxes its tension, in vain; the lens, for want of elasticity, does not swell out. Therefore the remedy for presbyopy is the use of convex glasses, not habitually, not in looking at distant objects, but only in looking at or imaging near objects. The putting on of convex glasses does not make the presbyopic eye normal, as the use of concave glasses makes the myopic eye; therefore they can not be worn habitually. In looking at near objects, it uses glasses;

in looking at distant objects, the glasses are removed. Myopy is a structural defect; presbyopy is a functional defect. One is a defect of prearrangement of the instrument; the other is a loss of power to adjust the instrument. To compare with the camera again: the presbyopic eye is like a camera which was originally arranged for distant objects, and by drawing the tube could be adjusted for near objects also, but, through age and misuse and rust, the draw-tube has become so stiff that the apparatus for adjustment no longer works. It still operates well for distant objects, but can not be adjusted for nearer objects. If we desire to image a near object in such a camera, obviously we must supplement its lens with another convex lens.

From what has been said it is evident that the myopic eye does not improve with age, and finally become normal, as many suppose. Myopic persons continue to wear glasses of the same curvature until sixty or seventy years of age. I have never known a myopic person who discontinued the use of glasses as he grew older. The same change, however, takes place in the myopic as in the normal eye, i. e., the loss of adjustment. In all young eyes there is a range of adjustment between a nearer and a farther limit; in the normal eye it is between five inches, near limit, and infinite distance, the farther limit (if limit it can be called); in the myopic eye the nearer limit may be two inches, the farther limit four inches, or it may be between three and six inches, or four inches and one foot, according to the degree of myopy. Now, with advancing age, the nearer limit, i. e., the limit of adjustment, recedes. In the normal eye it is first eight inches, then one foot, then three feet, etc., until, when adjustment is entirely lost, it reaches the farther limit, and there is but one

distance of distinct vision; but the farther limit, i. e., structural limit, does not change. So also in the myopic eye, with advancing age, the nearer limit or limit of adjustment recedes, but not the farther limit or structural limit. This remains the same. But, as this was always too near for useful vision, glasses must still be worn. Thus it is evident that myopy and presbyopy may exist in the same individual.

In extreme old age, when the tissues begin to break down, it is probable that some flattening of the eye may take place. To such persons it would be necessary to wear convex glasses, even for distant objects. But this is not ordinary presbyopy. In fact, it is probable that most of such cases belong to the next category.

Hypermetropy. We have dwelt on the two most common defects of the eye, but there are others less common, which must be briefly characterized. Hypermetropy is the true opposite of myopy. Like the latter, it is a structural defect, but in the opposite direction. In this case the lens is not sufficiently refractive for the length of the chamber, or the receiving screen is too near (at S'S', Fig. 7) for the refractive power of the lens. Therefore the focus of parallel rays is behind the retina in a passive state of the eye. The hypermetropic eye when young usually sees well at a distance, but not near at hand, and therefore it is apt to be confounded with presbyopy. The reason is, that a slight adjustment adapts the eye for perfect retinal image of distant objects; but the near limit of its range of adjustment is much farther off than in the normal. When, however, the hypermetropic eye loses its power of adjustment with age, then even distant objects can not be seen distinctly. Such persons, therefore, while young, should habitually wear slightly convex glasses,