Licence: Public Domain Mark
Credit: On conical cornea / by James H. Pickford. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
23/50 (page 17)
![the lens, I have supposed in Fig. I. and Fig. II., the latter to have been removed, and the eye, therefore, to be in the condi- tion in which it would have been, had the patient submitted to its extraction or solution for the cure of that defect of vision oc- casioned by the increased refractive powers of the conical cornea. In the case before us its presence would only have the effect of augmenting the undue convergence of the rays. I have not taken into calculation the refraction of the rays by the vitreous humour, as this is foreign to my object, but have allowed them to pass on in straight lines. It may be well, notwithstand- ing, to keep in view that the index of refraction of the cornea and aqueous humour, taken together, is 1*337, or, the sine of in- cidence and refraction as four to three ; that the mean refractive index of the crystalline lens is ] '384; and the refractive index of the vitreous humour, 1-339. Neither must it be forgotten, that, in the disease under con- sideration, the sphericity of the cornea, so to speak, is totally lost, saving so much of its obtuse apex as approximates to the form of the healthy cornea, and that we have no longer to treat an ellipsoid of revolution, but a plane surface, the slant sides of the cone. In the annexed diagram. Fig. I., let ABC represent the sclerotic coat of the eye, magnified four diameters. DD the healthy cornea. EE the sides of the corneal cone, subtending with the apex F an angle of ninety-two degrees. FB the axis of the eye. GG the iris. L a ray coincident with the axis of the eye FB. HIJK kj ih are rays parallel thereto. Kk traverse the corneal cone at the same distance from its apex F, and in- tersect the line of its axis at the point M, decussate and pass on to Qq. ' Jj intersect the same line at N, decussate and pass on to Rr. li at O, decussate and pass on to Ss; and Hh at P, decussate and pass on to Tt. In Fig. II. let ABC, as before, represent the sclerotic coat of the eye, magnified four diameters. DD the healthy cornea. c](https://iiif.wellcomecollection.org/image/b21950489_0023.jp2/full/800%2C/0/default.jpg)
