The refraction of the eye; including a complete treatise on opthalmometry; a clinical text-book for students and practitioners . Adjustable bracket for the Keidophthalmometer. Hardys Ophthalmometer2 The first ophthalmometer was designed by Helmholtz, in thefirst half of the present century; but the principle involved thereinwas not reduced to practical utility until 1884, when Doctors Javaland Schidtz, in Paris, designed the present model. The principleon which it is based is tin- measurement of corneal curves by means ofreflected images viewed through a telescope. 1 University Medinil Magazin
The refraction of the eye; including a complete treatise on opthalmometry; a clinical text-book for students and practitioners . Adjustable bracket for the Keidophthalmometer. Hardys Ophthalmometer2 The first ophthalmometer was designed by Helmholtz, in thefirst half of the present century; but the principle involved thereinwas not reduced to practical utility until 1884, when Doctors Javaland Schidtz, in Paris, designed the present model. The principleon which it is based is tin- measurement of corneal curves by means ofreflected images viewed through a telescope. 1 University Medinil Magazine., July, 1899. 2 Description taken from Hardys catalogue. 402 THE REFRACTION OF THE EYE Images reflected from curved surfaces will vary in size accord-ing to the radius of curvature of those surfaces, when the objectsreflected therefrom are uniform in size and distance. For in-stance, a circle 20 cm. in diameter, placed at 28 cm. distance,would produce a larger reflected image on a cornea which hasa long radius of curvature than would be produced on one with a. Fig. I. shorter radius. So that, if it were possible to measure the size ofthe images of the above object reflected from a cornea, the exactradius of curvature of that cornea could be calculated. Or, con-versely, if an image of a given size, say 3 mm., is reflected from thecornea, the curvature of the latter can be calculated from the size ofthe object required to produce 3 mm. at 28 cm. distance. It is evi-dent, therefore, that the problem to be solved in constructing an APPENDIX 403 ophthalmometer has been how to either measure the size of theimage reflected from the cornea, or to obtain on the cornea an imageoi a fixed size, Bay 3 mm*, in diameter. - The device employed accomplished both of these results, and isshown in Bection in Fig. 1. •It consists of a telescope T, to which are attached arcs car-rying sliding targets -A and •!», called mires. The telescopehas a combination of lenses such that a surface, when
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