The Croonian lecture : in which the power that enables the eye to see distinctly at different distances is considered / by Everard Home.

  • Home, Everard, Sir, 1756-1832.
Date:
[1795]
Catalogue details

Licence: Public Domain Mark

Credit: The Croonian lecture : in which the power that enables the eye to see distinctly at different distances is considered / by Everard Home. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    that in an achromatic object glass we obtain in a less perfect manner, by proportioning the radii of curvature of the different lenses. In the eye, the correction seems perfect, which in the object glass can only be an approximation, the contrary aber- rations of the lenses not having the same ratio; so that if this aberration be perfectly corrected at any given distance from the centre, in every other it must be in some degree im- perfect. Pursuing the same comparison : In the achromatic object glass,we may conceive how much an object must appear fainter from the great quantity of light lost by reflection at the sur- faces of the different lenses, there being as many primary re- flections as there are surfaces; and it would be fortunate if this reflected light was totally lost. Part of it is again reflected towards the eye by the interior surfaces of the lenses, which by diluting the image formed in the focus of the object glass, makes that image appear far less bright than it would other- wise have done, producing that milky appearance so often complained of in viewing lucid objects through this sort of telescope. In the eye the same properties that obviate this defect, serve also to correct the errors from the spherical figure, by a regular diminution of density from the centre of the crystal- line outward. Every appearance shews the crystalline to con- sist of laminse of different densities; and if we examine the junction of different media, having a very small difference of refraction, we shall find that we may have a sensible refrac- tion without reflection : now if the difference between the contiguous media in the eye, or the laminae in the crystal- line, be very small, we shall have refraction without having B2