Abstract
In a previous report 1 the origins and developments of the present systems of numbering ophthalmic prisms were discussed, and attention was called to three important facts: The power of prisms frequently departs unjustifiably from reasonable manufacturing standards. Different units of power are used, but the unit used is rarely indicated on the label. Enormous errors are introduced in using strong prisms unless careful precautions are taken respecting the position in which the prism is held and the unit used. Both the Dennett 2 system of utilizing the prism centrad (▽) as a unit and the Prentice system 3 of utilizing the prism diopter (△) as a unit require that all refractive effect must take place at one surface. This condition is violated more frequently than it is observed in clinical practice. I have watched many ophthalmologists, teachers as well as students, in making their measurements of squint, and the
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