By Double Images.—In studying the laws of refraction fraction as applied to the eye, it occurred to me that two opposite peripheral beams of light in the pupil could be made available in determining the posterior focal point of the eye relatively to the bacillary layer of the retina. This can be easily accomplished by using a disc or card with two small perforations in it, large enough to give distinctness of vision, and close enough together for both to fall within the area of the pupil. In order to prevent lapping on the retina, it is necessary to use a point of light for observation, which should be about twenty feet distant from the observer. A pin hole in a card over a candle, lamp or gas flame, or over a window against the sky; or, better still, as the room does not have to be darkened, a small
MURRELL TE. DETERMINING ERRORS OF REFRACTION BY DOUBLE IMAGES AND BY PARALLAX.. JAMA. 1885;V(11):284–285. doi:10.1001/jama.1885.02391100004001a
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