_Messrs. Ross_ have made many important improvements and changes in the construction of their several series of achromatic objectives; the calculations and formulæ for which were made exclusively for them by Dr. Schrœder. The list is too long to quote, but most of these lenses are of a high-class character, and work with admirable precision. Among the best of their objectives, I can commend a 1-inch of 30° and two oil-immersions, a 1/8-inch of 1·20 and a 1/12-inch of 1·25 numerical aperture, each of which bear the highest oculars equally well; a good test, as I have always maintained, of excellence. Their 1/10-inch has a somewhat larger aperture, and therefore shows a fine image of the podura scale. The finish of Ross’s several series of objectives fully maintains the high character and reputation of this old-established firm of opticians. _Messrs. R. and J. Beck_ have bestowed great attention upon the improvement of their dry-objective series, much in demand for histological work, especially among the students of city hospitals, who usually commence their pathological work with the cheaper forms of objectives. In that case an inch objective of about 25° air angle, a 1/2-inch of not less than 40°, and a 1/4-inch or 1/5-inch magnifying from 50 to 250 diameters, is quite sufficient for most of their work. For bacteriological research, Messrs. Beck supply a 1/6-inch immersion taken from a series, having a high aperture and a better finish at a moderate price. Their 1/10-inch immersion has in my hands proved a serviceable power for bacteriological research; it requires a good sub-stage illuminating achromatic condenser to obtain the best results. [Illustration: Fig. 123.--1/6-in. English Combination, largely used.]