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—The principal use of the ophthalmoscope is to enable us to study the interior of the eye. The use of its lenses for the measurement of refraction is of much less importance. In most of the forms of the instrument employed in this country its principal use has been somewhat sacrificed to assist its less important function. To measure refraction with the ophthalmoscope it is necessary by using a large sight-hole to exaggerate the blurring caused by imperfect focusing. For the other uses of the ophthalmoscope this blurring should be reduced as much as possible. It can not be entirely corrected by lenses, because in all eyes, with the pupil dilated as in a dark room, there is irregular astigmatism which no lens can correct. Such blurring is reduced directly in proportion to the reduction in the size of the sight-hole; hence, the most distinct view of the
JACKSON E. A PRACTICAL OPHTHALMOSCOPE.. JAMA. 1896;XXVI(18):857–859. doi:10.1001/jama.1896.02430700009002a