Fig. 123.—1⁄6-in. English Combination, largely used.
Messrs. Watson and Sons have much enhanced their reputation by the marked improvement lately brought about in the manufacture of their whole series of objectives. This probably is chiefly due to the introduction of the Jena glass into their manufacture, and which has enabled them to give increase of aperture to one series in particular, that of the para-chromatic, all of which in consequence are of very high quality. It is difficult to particularise their several objectives, the whole having special features in proportion to their magnifying powers, while much care seems to have been bestowed on them for the elimination of residual colour. A 1⁄8-inch with correction collar is comprised of a single deep and rather thick front lens, plano-concave flint, and double convex-crown for the middle and triple combination for the back, the latter consisting of two crown lenses cemented to a dense flint ([Fig. 124]) drawn to scale of 5-1, with lined portions intended to represent the flint, and white the crown glass lenses of the combination. The initial magnification of this objective is 83 diameters, and the numerical aperture ·94. This superior objective can be had for the small sum of £2. Another remarkably useful and cheap objective, their 1-inch numerical aperture 0·21, consists of two achromatic systems forming the front and back with the separation between them of about half an inch, and may also be especially recommended for students’ work.
In the accompanying diagram the lenses are drawn on too large a scale, and therefore the distance between the two combinations should be much greater.
Among the more useful of Watson’s series, the 1-inch, the ½-inch, and the 1⁄6-inch, together with the 1⁄8-inch dry-objective, and a 1⁄9-inch, will be found the most serviceable.
Fig. 124.—Watson’s 1⁄8-in. Objective Para-chromatic Combination, scale 5-1.
Fig. 125.—Watson’s 1-in. Achromatic Combination.
Messrs. Baker have their own series of objectives, most of which are so very nearly allied to those of the continental opticians; and what has been said of Zeiss’s and Leitz’s objectives may be taken to apply also to Baker’s, who have an established reputation for their histological series, all of which are well suited for students’ and class-room work.