Soon after the immortal invention of Helmholtz, I found the ophthalmoscope very useful in detecting slight degrees of conical cornea. For this purpose the concave mirror only is to be used without a convex lens. On turning the mirror so as to throw light at different angles, the side of the cone opposite to the light is dark.
Reference: Bowman W. On conical cornea and its treatment by operation. Ophthalmic Hosp Rep and J R Lond Ophthalmic Hosp. 1859;9:157.
A look at the past . . .. Arch Ophthalmol. 1999;117(1):89. doi:10.1001/archopht.117.1.89