Elements of human physiology (1907) Elements of human physiology elementsofhumanp05star Year: 1907 SPECIAL SENSES 559 tion is corrected in optical instruments by using two diHerent kinds of glass. In the eye it is uncorrected. Hence it is that a blue light and a red light at the same distance from the eye appear to be unequally distant; the red light, re- quiring greater accommodation than the blue, appears to l)e the nearer of the two. The error in most cases is so slight that we do not notice the chromatic frmges under normal circumstances. Myopia.—The normal or emmetropic eye is so constru
Elements of human physiology (1907) Elements of human physiology elementsofhumanp05star Year: 1907 SPECIAL SENSES 559 tion is corrected in optical instruments by using two diHerent kinds of glass. In the eye it is uncorrected. Hence it is that a blue light and a red light at the same distance from the eye appear to be unequally distant; the red light, re- quiring greater accommodation than the blue, appears to l)e the nearer of the two. The error in most cases is so slight that we do not notice the chromatic frmges under normal circumstances. Myopia.—The normal or emmetropic eye is so constructed that, when the ciliary muscle is relaxed, parallel rays are Fio. 2-50. Diagrams of course taken by parallel rays in entering normal (emmetropic) eye {A), hypermetropic eye (B), and myopic eye (C). brought to a focus on the retina. If the eyeball be longer than usual, the parallel rays will come to a focus rather in front of the retina, so that it will be impossible for a clear image of distant objects to be formed on the retina. Objects at a certain small distance from the eye will be brought to a focus on the retina without any effort of accommodation. People with eyes of this description are said to be myopic or short- sighted. Under these circumstances concave spectacles are necessary, in order to form a distinct retinal image of distant objects.
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