convenient one, but is only one of several which may be used, according to the inclination of the oculist.

Astigmatic eyes are often poor eyes, and vision is, in some cases, only moderately improved by correctingglasses at first, although in a certain proportion of cases marked improvement may be observed after several months.

UNEQUAL REFRACTION OF THE EYES (ANISOMETROPIA).

It is not uncommon to find a difference in the refraction of the two eyes. One eye may be emmetropic and the other myopic, hyperopic, or astigmatic, or there may be different degrees of ametropia in the two eyes.

If the difference is small, it is best to correct the error of each eye. If, however, there is great disparity of the refractive conditions, much difficulty may be experienced by the patient in trying to correct both. In such cases there is usually found very marked anomalous tendencies of the ocular muscles which seriously complicates the situation. Before glasses perfectly adapted to each eye can be used with comfort in such extreme cases, it is necessary to establish muscular equilibrium, after which there is a better prospect of harmonious action of the two eyes.

TREATMENT OF PRESBYOPIA.

It has already been shown that presbyopia consists of the gradual recession of the near point of clear vision toward the distant point. At the age of fortyfive, owing not to the flattening of the eyeball, as has

been popularly supposed, although a slight change in the length of the axis of the eye actually occurs, but to the loss of elasticity of the crystalline lens, the emmetropic eye finds some difficulty in reading fine type, especially in the evening. According to Donders, the near point of clear vision in the emmetropic eye is, at the age of ten, two and two-thirds inches in advance of the eye; at twenty it is three and a half; and at forty a little more than eight inches removed from the front of the eye. There is, then, no absolute point where the change to presbyopia commences, and the selection of a point which shall be regarded as presbyopia is entirely arbitrary, based upon the needs of the great majority of those who require glasses on account of advancing age. Donders fixes the point of commencing presbyopia as that at which the near point has receded to more than twenty-two centimetres, or about nine inches in front of the eye.

If

Adopting this as the commencement, we determine the degree of presbyopia in a very simple manner. we bring small letters (No. 0.5) toward the eye until we find the nearest point of clear vision, we calculate the difference between this point and the point of commencing presbyopia; thus:

Presbyopia = -1; n in the formula represents the near point ascertained. If n = 12 inches, then the formula reads:

Presbyopia then, and a glass of thirty-six inches focal length is required to bring the near point to nine inches.

Substituting the metrical system, in which a lens with the focal value of nine inches is represented by 4.50 D, and one of twelve inches by 3.25 D, and our formula will be

The following table indicates the lenses which, according to Donders, are required for presbyopia of emmetropia at different ages:

It must not be supposed that every pair of emmetropic eyes will find these glasses exactly suited to the necessities of close work. The distance at which work is to be done is to be considered in the selection of the glasses; thus a public speaker who reads his notes while speaking will require glasses, other circumstances being equal, weaker than one who works at a desk. Again, vision of emmetropic eyes, through the influence of muscular insufficiences or other causes, is not unfrequently less than the normal standard. Hence, glasses must be adapted especially to the individual.

It is evident that if hyperopia, myopia, or astigmatism exists, either condition must be taken into consideration. If hyperopia exists, the amount must be added to the degree of presbyopia shown in the table. If myopia is present, it is to be deducted; and

in a case of astigmatism, the value of the correcting cylinder is to be added or subtracted.

In determining the glasses required in an individual case we employ test-types. Those in most general use for this purpose are Snellen's. These types are so graduated as to represent the greatest distance at which they should be read. Thus the smallest (see page 169), 0.5 D, should be read at one half metre (eighteen inches), the fourth at one metre (forty inches), and the sixth at 1:50 metre.

In testing presbyopia, we first correct the ammetropia by placing the proper glasses in the trial-case. The patient is then required to read the smallest types which can be read, and the nearest and farthest points are noted. In practice it is advisable not to force the eyes to read the smallest type at nine inches, but at a somewhat greater distance; twelve inches may be accepted. If the patient, in holding the card at twelve inches in front of the eyes, is able to read the types marked 1.25, and none smaller at this distance, we may place glasses of 1.25 D in the frames. It will now be found that No. 50 can be read at twelve inches. The type is then to be removed to the distance indicated by the number 0.50 (eighteen inches), and if it can still be read at this distance, there is a reasonable amplitude of accommodation, and the glasses are not too strong.

If the patient is quite myopic, the fully-correcting concave glasses may be left out of the trial-frames, and the weaker glass, which will permit the small types to be read at the specified distance, may be chosen.

In concluding this sketch of the subject of refraction and accommodation, it will not be out of place to call attention to some general points of interest which are incidentally related to the subject.

Ametropic eyes eyes varying from the ideal standard-are very common. It is an ordinary occurrence for the oculist to find persons who believe that they are blessed with the best of vision, and who boast of its excellence, to have really quite defective eyes, and perhaps very indifferent vision. To assume that one has excellent eyes because the name of a distant steamboat can be read by the possessor when others do not read it, or because the letters of a sign-board are seen when one's neighbor does not read them, is to presume that all the other persons who may be looking at these objects have perfect eyes, and that their attention has been equally directed to the object. Such tests prove nothing, and should not lead one to assume a perfection which may not exist.

A very popular error is the supposition that one must have "strong" eyes because their possessor is able to see small objects better than others, and such. a person is likely to boast of the ability of one or both parents to read without glasses until at an advanced age. It has been shown already that, when people who have passed the age of fifty are able to read without glasses, it is an indication, not of perfect eyes, but of myopia; and when people see minute objects better than usual, we also conclude that they are nearsighted.

Many people who have refractive or muscular disa