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The ordinary trial frames and test lenses seldom conform to the modern standards of the skilful medical refractionist. In hospitals and dispensaries they are especially faulty, and often ludicrous. It is most important that the trial frame shall be firmly and snugly placed on the sides of the nose and adjacent cheeks, in approximately the position in which the prospective spectacles or eye-glasses will be worn. In this respect all frames fixed to mechanical instruments, such as optometers, are at fault, often clumsy, and are signally deficient for accurate testing.
The prime requisites for a trial frame are rigidity, lightness, proper position on the face, and ease of general adjustment and manipulation of the trial lenses. There is also a necessity for individual adjustment of the two sides, to permit perfect binocular centering.
The test lenses, carefully centered, should be placed before the eyes, properly inclined, and just clearing the
Pyle WL. THE TRIAL FRAME FOR TEST LENSES, WITH DEMONSTRATION OF A NEW MODEL. JAMA. 1918;71(12):966. doi:https://doi.org/10.1001/jama.1918.26020380002002a
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