To present the subject in a logical manner, permit me to review in a general way the refractive quality of the normally and abnormally constructed eye, and the adjustment of its external muscular system.
The eye may be emmetropic, hypermetropic, myopic, or astigmatic.
Emmetropia is, of course, the ideal condition, and it may be defined as that refractive condition in which distant or parallel rays of light are focused on the retina without any effort on the part of the accommodative muscle. Such an eye, in regarding distant objects, i. e., objects at or beyond 20 feet, is merely a camera obscura, in which the focus is formed on the retina. Such an act, under such conditions should be, in the natural order of things, achieved without discomfort. (Fig. 1.)
The hypermetropic eye is an eye that is shorter in its anteroposterior diameter than the emmetropic eye, and hence, given
CLAIBORNE JH. THE NERVOUS SYMPTOMS PRODUCED IN CHILDREN BY UNCORRECTED REFRACTIVE AND MUSCULAR ERRORS.. JAMA. 1904;XLIII(24):1754–1760. doi:10.1001/jama.1904.92500240002d