The refraction of the eye : including a complete treatise on opthalmometry; a clinical text-book for students and practitioners / By A. Edward Davis; with one hundred and nineteen engravings, ninety-seven of which are original.

Davis, Achilles Edward, 1866-1941.

Date:: 1900

Credit: The refraction of the eye : including a complete treatise on opthalmometry; a clinical text-book for students and practitioners / By A. Edward Davis; with one hundred and nineteen engravings, ninety-seven of which are original. Source: Wellcome Collection.

Provider: This material has been provided by the Harvey Cushing/John Hay Whitney Medical Library at Yale University, through the Medical Heritage Library. The original may be consulted at the Harvey Cushing/John Hay Whitney Medical Library at Yale University.

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$each corresponds to ;i displacemenl of fche eye-piece to the extent of 1 nun., each millimeter of displacemenl corresp Ling to a change in the observer's eye of ;>> diopters. For example, when the pin on the eye-piece stands at zero an emmetropic observer should see the cross-threads in the telescope plainly, while a myope of 3 diopters would have to turn the pin one graduation to the right, and a hypermetrope of 3 diopters would have to turn the pin one graduation to the left in order to see the cross-threads distinctly! </■) The two objectives are exactly alike. Each has a diameter of 40 mm., or 1.6 inches, and a focal distance of 280 nun., or 11.2 inches. Thej are the most perfect lenses made, being both achromatic and aplanatic. They arc so placed in the barrel of the telescope that the crown of one is turned toward the observed eye, and the crown of the other toward the observer. By this means the flints of the two objectives are kept opposed and next to the bi-refract- ing prism. (c) The cross-threads are two very fine wires stretched at right angles to each other across the barred of the telescope about 1 inch in front of the eye-piece and about 11.2 inches behind the first objective, L'. They are there simply to show when the eye-piece is in proper focus with the objectives. (*/) The bi-refracting prism (Wallaston) placed between the two objectives is in fact two prisms, placed apex to base and base to apex. In this country, at least in the instruments man- ufactured by Meyrowitz.and Georgen, these prisms are made from the best mountain crystal quartz. They possess the power of doubling objects, that is, are bi-refractive, if ground in a certain direction with regard to the axis of the quartz. The amount of the deviation or doubling produced by each prism depends on the angle at which it is ground. One of the prisms is ground diagonally with the grain of tin- quartz (a, Fig. 3), while the other is ground at right angles$