1851 Great Exhibition: Official Catalogue: Class X.: J. B. Reade
254A. READE, Rev. J. B., F.R.S., Stone Vicarage, Aylesbury — Inventor.
Positive solid eye-pieces.
The solid eye-piece (see the following cut) consists of two double convex lenses, c and e of crown glass, with an intermediate double concave lens, d, of flint, having the contact surfaces cemented together. The cap, b, contains a small eye-hole, a. Its novelty consists in its construction, which secures a large and flat field of view, together with the removal of spherical and chromatic aberration. In consequence of the purity of its achromatism, the webs of the transit instrument and micrometer are seen as fine black lines, and hence it is found by experiment that observations are made more perfectly than with the common positive eye-piece which is not achromatic. No light is lost, as in the usual construction, by inner reflections, and there is no formation of the false image or "ghost" of planets and the brighter stars. From the following data the curves of the lenses may be determined for a given focal length:—
Index of refraction of flint … 1.600
Index of refraction of crown … 1.523
Ratio of dispersive powers … 0.657
Thickness of flint lens … 0.775 xf.
where f = whole focal length for parallel rays.