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Most oculists are loath to prescribe prisms for wear, either alone or in combination with a lens correcting ametropia. Few patients can tolerate the aberrations produced by prisms. Aside from the increasing deviation that a prism produces when the eye does not look through it in a position of minimum deviation, and aside from the distortion arising from the fact that when an object of some size is seen through a prism the light rays reaching the eye from the peripheral parts of the object
strike the face of the prism at more acute angles than the rays from the more central parts of the object and thus are refracted more, there is still another type of distortion which is, I believe, largely responsible for the discomfort caused by the small angle prisms that oculists may prescribe. I do not believe that the first two types of distortion are