1851 Great Exhibition: Official Catalogue: Class X.: John Thomas Goddard
274. GODDARD, JAMES THOMAS, 35 Goswell Street — Manufacturer.
Achromatic object-glass for a telescope of 9 inches aperture, and about 16 feet focus.
[The larger glass placed in telescopes, or that which is placed the farthest from the eye, is termed the object- glass. If this glass consists of a single lens, the image of a circular object will not be a perfect circle, as it ought to be, for such a lens will not refract all the rays falling upon it to a single point, and will cause an image in its focus to be both distorted and coloured; the former defect arises from the fact that no spherical lens will produce a perfect image; and the latter, from the unequal refrangibility of the coloured rays which, united, form a perfectly colourless image, and thus the image will be surrounded with several colours. The most important improvement in object-glasses was made in the year 1757, by Dollond. This was effected by making the object glass double, one portion being made of flint glass and the other of crown glass, of different refractive powers, which mutually correct each other, and thus give a pencil of light entirely colourless. Such object-glasses are called achromatic.—J. G.]