1851 Great Exhibition: Official Catalogue: Class X.: William Wray
309. WRAY, WILLIAM, 43 Havering Street, Commercial Road East — Inventor and Manufacturer.
A seven-feet achromatic telescope, four and a half inches in aperture, upon a new principle, in which the difficulty of obtaining large discs of flint glass is overcome by the employment of a solid substitute.
This substitute is a combination of a resinous substance, with a very highly refractive and dispersive essential oil, and its application is extremely simple and effective, as well as comparatively inexpensive. From its homogeneous nature, it would appear that a finer telescope might be constructed with lenses of this substance, than with lenses of flint-glass; for it has been found a matter of extreme difficulty to produce the dense flint- glass perfectly homogeneous, and free from striae. The substitute for flint-glass is highly transparent, of a pale straw colour, and is unalterable by light air, moisture, and moderate heat; it fuses at about 200 degrees, and is applied in a fluid state between crown-glass lenses. It becomes solid on cooling, and remains so at all atmospheric temperatures. Its refractive index and dispersive power allow a shallower system of curves than flint-glass usually does, and consequently a better correction of the spherical aberration. On account of the composition being in perfect contact with the two crown- glass lenses between which it is put, little light is lost by reflection at the four interior surfaces, which is not the case with the ordinary achromatic object-glass.
[1. An achromatic glass is a compound lens formed of two kinds of glass, which act differently upon the rays of light transmitted by them. The effect of their united action is to bring all the rays of light to a common focus, which ordinary single lenses will not effect. Objects seen through achromatic lenses are not surrounded with those fringes of colour which encircle them when examined by a single lens. Hence the term achromatic. In the case in question, the disc of flint-glass is substituted by a solid resinous substance.—R. E.]