The phenomenon of night myopia, wherein the eye becomes relatively nearsighted in dim light, was investigated in detail using high contrast grating test objects. Night myopia first appeared at the brightness level where rod vision began to take place and grew larger as the brightness was further reduced. At the lowest brightness investigated, the myopia attained a value of 1.5 to 2.0 diopters, depending upon the observer. Night myopia appeared when accommodation was prevented by an optical method and also when accommodation was paralyzed with homatropine. It was therefore concluded that accommodation was not a significant cause of night myopia in the observers examined.
The spherical aberration of the observers’ eyes was measured, and its effect upon the effective focal length of the eye was investigated with the aid of artificial pupils. Also studied were the properties of a simple glass lens having spherical aberration approximating that of the eye. All tests showed that night myopia, and its dependence upon the brightness level, is primarily a result of undercorrected spherical aberration of the eye. For some eyes, homatropine reduced night myopia slightly, but only to the extent that it reduced the spherical aberration. A review of the literature is included.
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