William Herschel's Contributions

As representative of the prices Herschel charged for his reflectors, a Newtonian model of 6½-inch diameter and seven feet 16 focal length sold for 100 guineas (30 guineas for the optical parts).

Another 8.8-inch Newtonian, 10-foot focus, cost 200 to 300 guineas. Herschel advised buying two mirrors for this latter instrument (which probably accounts for the variable price) so that one could be used while the other was being repolished!

His talents were not confined to the making of fine specula; he also made his own eyepieces, some of which were truly remarkable. His frequent references to the use of magnifications of some 7,000 on his 6½-inch reflector occasioned some speculation and controversy among the English astronomers, but his claim appears to have been justified by the discovery, comparatively recently, of some very tiny eyepieces made by Herschel.

Among his effects at Slough, W. H. Steavenson found several of these eyepieces, varying in focal length from about 1/16" downward. The smallest of these was examined in a microfocometer, and found to have a focal length of 0.011". It was bi-convex, about 1/45" in diameter, and 1/90" in thickness. It was tried out on a 6-inch refractor, and performed as creditably as its power would permit, but its field in that instrument was only about 20 seconds of arc in diameter. If Herschel actually used this eye- piece on his 85.2" focal length reflector, it would have
given a magnification of  7,668.

The Achromatic Refractor