English Immersion and Dry Objectives.

The homogeneous immersion system met with its earliest as well as its staunchest advocates among English opticians. Among its more energetic supporters were Messrs. Powell and Lealand, who were the first to construct a ¹⁄₈-inch immersion objective on a formula of their own, and which was found to resolve test-objects not before capable of resolution by their dry objectives. This encouraged them to make a ¹⁄₁₆-inch, acquired by Dr. Woodward for the Army Medical Department, Washington, and subsequently a ¹⁄₂₅-inch; neither of which surpassed their ¹⁄₈-inch in aperture, and a new formula was tried in the construction of their first oil-immersion objective. This had a duplex front, and two double backs; but even this did not quite accomplish what was expected of it, and another change was subsequently made; the anterior front combination became greater than a hemisphere—a balloon-lens. This at once gave an increase of aperture to a ¹⁄₁₂-inch objective of 1·43 numerical aperture. After some few more trials a more important change of the formula took place. The front lens was made of flint-glass, and the combination took the form represented in diagram ([Fig. 122]). This, on an enlarged scale, represents Powell’s ¹⁄₁₂-inch numerical aperture 1·50. It is a homogeneous apochromatic immersion of high quality and very flat field. It will be noticed that in this combination the four curves of the lenses are very deep compared with those of other opticians.

Fig. 122.—Powell and Lealand’s ¹⁄₁₂-in. Oil-immersion Objective, drawn on a scale of 6-1.

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 ½-inch of not less than 40°, and a ¼-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.

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.

Messrs. Swift and Son have a new series of objectives, semi-apochromatic and pan-aplanatic, most of which are excellent in quality and show increased flatness of field together with that of achromatism; the index of refraction in each series having been correctly determined together with exact radial focal distance, thus affording more available aperture. I may select for special commendation their ¹⁄₁₂-inch £5 5s. homogeneous immersion objective, which is in every way suitable for bacteriological work; its definition is very good, as is seen in a micro-photograph of podura scale, given further on. Their dry ¹⁄₆-inch can be had for £1 16s.—a marvel of cheapness. Of their general series the most useful for histological work are the ½-inch, the ¹⁄₃-inch at £1 12s., and their ¹⁄₅-inch of numerical aperture 0·87 at £3.

Mr. Pillischer, of Bond Street, has manufactured many excellent objectives. A fine homogeneous oil-immersion ¹⁄₁₂-inch numerical aperture 1·25 is worthy of special notice; it will be found suitable for bacteriological work; it has fine definition with a considerable amount of penetration.

A more intelligent idea of the magnifying power of the objective combined with the eye-piece will be gained by consulting the table given below; precision in this respect has long been a desideratum with microscopists.