28 PRESBYOPIA.
in the proper relation to the age of the patient. Paralysis of accommodation is partly thus excluded, partly, also, by the beha- vior of the pupil. As a rule, the pupil is contracted in presbyopia and dilated in paralysis of accommodation.
The treatment consists in substituting convex lenses for the lost power of accommodation. So soon as presbyopia causes incon- venience which is relieved by the use of weak convex lenses {-^ to 4^), such spectacles should always be used for work. It is useless, by straining the eyes, to attempt to postpone the use of glasses, and it is equally useless to a'dopt them so long as, in spite of the removal of the far point, continuous reading or writing causes no inconvenience.
The strength of the glasses to be prescribed depends upon the nature of the patient's occupation. The farther his near point has receded beyond the distance of the objects on which he uses his eyes, the stronger the glasses which he requires. For reading and writing it generally suffices to bring the near point up to a distance of from 12 to 15 inches. In simple, uncomplicated presbyopia, with fair acuteness of vision, the choice of proper spectacles may usually be left with the patient. From time to time, as the near point gradually recedes from the eye, somewhat stronger glasses must be chosen. For healthy eyes it is better to use uncolored glasses. The blue ones, which have become per- haps too fashionable, are at first very agreeable, but since they deprive the retina of the normal stimulus of white light, the eye becomes too sensitive; hence having once begun, it is necessary to continue wearing them.
DETERMINING ACUTENESS OF VISION.
We have said that in making the diagnosis of presbyopia it is necessary to determine the acuteness of vision ; we must now con- sider the method of doing it. A sufficient judgment may be formed by trying the acuteness of vision by test type of varying sizes, such as Jaeger's. In many respects, however, Snellen's test letters are better. They are more exact, and the results are more easily compared with each other. These letters increase in size according to a definite proportion.
The most suitable objects appear to be black letters, of sizes so arranged that they are seen plainly by the normal eye under
