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32 MYOPIA. adjust itself for these distances. Since, however, we wish to de- termine the optical adjustment of the eye when its accommodation is fully relaxed, we have the important rule that only the weakest concave lens with which distant vision is possible gives the proper expression for the position of the far point. One ought not to trust the subjective impressions of the patient ; his simple state- ment that he sees better, is not sufficient unless in connection with it the reading of the test letters shows a perceptible im- provement in vision. Weaker and weaker glasses should be tried till one is found which diminishes the sharpness of vision ; the limit is then passed.

If acuteness of vision be diminished, as is usually the case in the high grades of myopia, then the weakest concave lens which gives the best attainable vision must be regarded as giving ap- proximately the correct expression for the degree of myopia.

If the position of the far point requires the employment of a moderately strong concave lens, say — -^ or more, there is a very simple method of determining whether it is too strong or not. Move the lens, and of course at the same time its negative focal point, gradually away from the eye ; if with the lens close to the eye its negative focal point coincided with the far point, then upon withdrawing the lens its focal point will be removed beyond the far point, and vision loses its distinctness; if upon Avithdrawing somewhat the lens vision is as distinct or even more distinct than with the lens held close to the eye, it shows that the concave lens is too strong. In the case of strong concave lenses the distance between the eye and the lens should not be neglected ; if, for instance, it be ^ inch and the far point lie at 3 inches, then will the negative focal point of a lens — ^^ coincide with the far point.

The degree of myopia is determined by the position of the far point; if it lie at 16 inches the degree of myopia is expressed by M^; if it lie at 3 inches, by M^. That concave lens whose negative focal point corresponds with the far point is called the neutralizing lens. The far point of an eye provided with a neu- tralizing lens lies, as it does in the emmetropic eye, at an infinite distance.

Formerly the optical cause of myopia was thought to be too great a curvature of the cornea. By ophthalmometric measure-