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January 4, 1896


Author Affiliations


JAMA. 1896;XXVI(1):28-29. doi:10.1001/jama.1896.02430530028001m

It is not intended here to enter upon the thorny discussion of one-fourth degree prisms, but to call attention to the frequent errors of one-half degree or more in prisms of low degree, bearing the same number, and reposing in the same trial-set, the statements of the manufacturer to the contrary notwithstanding.

However accurate the trial-lenses may be, little thought seems to be given by the manufacturer to the accuracy of the prisms. Frequently the prisms are rotated, and the base apex line wrongly marked that the prisms may neutralize. The effect of these errors upon diagnosis is obvious.

While the difficulty of grinding prisms to a predetermined refracting angle has necessitated the measurement of each glass separately to determine the angle of deviation, with which alone the ophthalmologist is concerned, nothing but expensive and elaborate appliances have been offered with the assurance of anything like exactness.

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