Page:Handbook of Ophthalmology (3rd edition).djvu/47

MYOPIA, CAUSES AND COURSE. 41 degree of myopia, choroidal changes which must be regarded as inflammatory. This, however, does not justify us in regarding the process as an inflammatory one from the beginning, and tlie atrophied crescent as tlie result of a sclerotico-choroiditis poste- rior.

Myopia may remain stationary through life, or at an advanced age it may show an apparent or actual decrease. The apparent decrease is due to the contraction of the pupil, by which the circles of diffusion are made smaller and the images of distant objects more distinct. An actual decrease of myopia may be caused by the senile changes in the lens before referred to.

In other cases myopia is rapidly progressive during a certain period of life, generally up to about the twenty-fifth year, and after that remains stationary. Finally, it may happen that myopia remains progressive through life,

The myope receives with the naked eye only indistinct, diffuse retinal images of all objects lying beyond his far point. In low degrees of myopia the indistinctness of distant vision is often not so great as to cause marked inconvenience, but the opposite is true in high degrees of myopia, where the retinal images of ob- jects only a few feet distant are very confused. If, moreover, the objects are insufficiently illuminated, as in the twilight, vision will be very imperfect.

The size of the circles of diffusion increases with the distance of the object and the size of the pupil.

Myopes are often better able than emmetropes to recognize very small objects, even in a poor light, since the jx)sition of the near point allows a closer approach of the object.

As the far point approaches the eye, the whole range of accom- modation approaches with it, so that without any lessening of the angle included by the accommodation the space covered by accom- modation is greatly diminished. If, for instance, with MyL the action of accommodation is as great as we have above assumed it to be in the emmetropic eye, that is ^=1, the position of the near point may be calculated from the formula -f -j- i:=^, which becomes ^2 + 4 = 3^- The whole region of distinct vision lies between the points 3 inches and 4 inches distant, while the

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