Ocular refraction and the shadow test . ness, and it is desired to place themin piles of twenty-four each, making the height of each pile threeinches. The simplest way to do this would be to count out twenty-four and stack them into one pile, as a measure, see A, figure 133. 164 OCULAR REFRACTION The pile B is too short, and the pile C too tall, as the dotted lineshows. Some must be taken from C, and more added to B. If someone should ask the height of the piles B and C as theywere originally, and say that the blocks in each must not be counted,nor the piles measured, the problem could be solv
Ocular refraction and the shadow test . ness, and it is desired to place themin piles of twenty-four each, making the height of each pile threeinches. The simplest way to do this would be to count out twenty-four and stack them into one pile, as a measure, see A, figure 133. 164 OCULAR REFRACTION The pile B is too short, and the pile C too tall, as the dotted lineshows. Some must be taken from C, and more added to B. If someone should ask the height of the piles B and C as theywere originally, and say that the blocks in each must not be counted,nor the piles measured, the problem could be solved by counting thenumber it was necessary to add to B, and the number it was requiredto take from C. These, compared to the known quantity in A, willserve as a basis from which to calculate the numbers in B and C, andthe height of the two original piles. Thus, in the practice of retinoscopy, if a certain lens value is re-quired to be imposed to focus emergent rays to a definite point, onemay calculate the unknown refraction of the Figure 133. Diagram to represent the meaning of the term, Working distance in retinoscopy. These propositions will no doubt seem simple enough to the stu-dent, the bringing to a focus of the emergent rays to a definite point,by the interposing of certain lenses under certain conditions; but, hemay, and doubtless will ask—How is one to know, when one observesan eye with the retinoscope, whether it is emmetropic, myopic orhypermetropic?—The answer is; the appearance of the luminouspupil varies according to the diflterent conditions of refraction. Thus far the theory of retinoscopy has been explained, the nextstep will be to take up acliialpractice and -norking conditions The study of retinoscopy has brought into use a simple devicecalled the model eye, which is a most valuable aid to the student, who RETINOSCOTY. 165 may practice upon it by the hour without its entering any protest offatigue. A description of it in the inventors own words will
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