OBITUARY

(Lat. obitarium.) A register in which are enrolled the names of deceased persons for whom obits are to be performed, and the days of their funeral. It is also used for the book containing the foundation or institution of the several obits in a church or monastery. In the former sense it is synonymous with necrology, in the latter with martyrology. OBJECT-GLASS (of a refracting telescope or microscope). The lens which first receives the rays of light coming directly from the object, and collects them into a focus, where they form an image which is viewed through the eye-gla . The excellence of an object-gla depends on the distinctne of the image which it forms. On account of the unequal refrangibility of the rays of light, it is nece ary, in order to procure a distinct image, to employ an achromatic combination of lenses, formed of substances having 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 gla and flint-gla ; the dispersive powers of which are respectively as 3 to 5. By combining a convex lens of crown-gla with a concave lens of flint-gla , havinK their focal distances in that proportion, an image would be formed free from colour, but it would not be free from aberration. The determination of the form of the compound lens which shall give the least po ible aberration for parallel rays is a problem which admits of exact 3 H 3 [s. 850]