OBJECT-GLASS (of a refracting tele scope or microscope). The lens which first receives the rays of light coming di rectly from the object, and collects them into a focus, where they form an image which is viewed through the eye-glass. The excellence of an object-glass de pends on the distinctness of the image which it forms. On account of the un equal refrangibility of the rays of light, it is necessary, in order to procure a distinct image, to employ an achromatic combin ation of lenses, formed of substances hav ing different dispersive powers, and of such figures that the aberration of the one may be corrected by that of the other. The substances chiefly used are crown glass and flint-glass ; the dispersive powers of which are respectively as 3 to 5. By combining a convex lens of crown glass with a concave lens of flint-glass, having their focal distances in that pro portion, an image would be formed free from color, but it would not be free from aberration. The determination of the form of the compound lens which shall give the least possible aberration for par allel rays is a problem which admits of exact calculation. The following are the dimensions found by Sir John Herschel for an object-glass of thirty inches focal length, tie convex lens of which was of common glass, the outside towards the object, and the concave lens of flint-glass on the side next the eye : radius of the exterior surface of the crown lens, 20.0364 inches ; radius of the exterior surface of the flint lens, 41.1887 inches ; radii of the interior surfaces, 10.1604 and 10'1613 When the lenses have the forms here in dicated, the focal lengths of each separ are in the direct ratio of their dis persive powers ; and the two inside sur faces have so nearly the same curvature that they may be ground on the same tool, and united by a cement to prevent the loss of light at the two surfaces.
Such are the forms indicated by theory; but the 'practical difficulties of forming a good achromatic object-glass, for a tele scope of large size, are so great that it often costs more than all the rest of the instrument. This, however, principally arises from the extreme difficulty of pro-: curing disks of flint-glass, above a cer tain size, sufficiently free from veins and imperfections as to be fit for the purpose.
No object-glasses of a larger size than seven inches diameter have been made of glass manufactured in England ; and, not withstanding the success of Fraunhofer at Munich, and of Guinaud in Switzer land, the procuring of flint-glass fit for object-lenses of a larger size seems to be still, in a considerable degree, a matter of accident. Fraunhofer executed a telescope for the Russian observatory at Dorpat, having an objeet-glass of 9 inches diame ter. Another was prepared by him for the King of Bavaria, of 12 inches diame ter. The object-glass of Sir James South's large telescope at the Campden Hill observatory is nearly 13 inches in diameter, and was executed in Paris of glass manufactured by Guinaud.
In the fine telescopes formerly con structed by Dollond, the object-glasses were composed of three lenses, the two exterior ones, being of crown-glass, and convex, and the interior of flint and con cave. This combination gives a more perfect correction of the spherical alprera lion ; but the advantage is more than balanced by the greater complexity of their construction, the risk of imperfect centering, and the loss of light at the six surfaces. They have accordingly been disused.
Various attempts have been made to dis pense with the concave flint lens, by the substitution of some other refractive sub stance. Dr. Blair found that the disper sion of crown-glass was corrected by a fluid lens, composed of a mixture of solu tions of ammoniacal and mercurial salts. He succeeded in making object-glasses in his manner, which at first gave promise of answering well ; but it soon appeared that they were not durable, the fluid undergoing some chemical change which entirely destroyed its virtue. Professor Barlow, of Woolwich, has also made nu merous experiments on this subject. His correcting lens is formed of the liquid sulphuret of carbon, inclosed between two disks of glass and a ring of the same material, the fluid being introduced at a high temperature. A telescope which he made on this principle had a single ob ject-lens of 7-8 inches, and the fluid lens was placed at the distance of 40 inches behind it. The performance of this tele scope was, however, far inferior to an or dinary one of the same dimensions, with the common double achromatic object glass. See LENS.