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CHAPTER VII |
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THE VARIABILITY OF THE REFRACTION OF
THE EYE
T
HE theory that errors of refraction are due to permanent deformations of the eyeball leads nat urally to the conclusion, not only that errors of refraction are permanent states, but that normal refrac tion is also a continuous condition. As this theory is almost universally accepted as a fact, therefore, it is not surprising to find that the normal eye is generally re garded as a perfect machine which is always in good working order. No matter whether the object regarded is strange or familiar, whether the light is good or im perfect, whether the surroundings are pleasant or dis agreeable, even under conditions of nerve strain or bodily disease, the normal eye is expected to have normal re fraction and normal sight all the time. It is true that the facts do not harmonize with this view, but they are conveniently attributed to the perversity of the ciliary muscle, or if that explanation will not work, ignored altogether.
When we understand, however, how the shape of the eyeball is controlled by the external muscles, and how it responds instantaneously to their action, it is easy to see that no refractive state, whether it is normal or abnormal, can be permanent. This conclusion is confirmed by the retinoscope, and I had observed the facts long before the experiments described in the preceding chapters had offered a satisfactory explanation for it. During thirty years devoted to the study of refraction, I have found
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