Fig. 121.—Zeiss’s Cover-glass Gauge.
COVER-GLASS GAUGE.
Zeiss has gone a step further to lay the microscopist’s ghost of the cover-glass. He invented a measurer ([Fig. 121]) whereby the precise determination of thickness of glass-covers can be obtained. This measurement is effected by a clip projecting from a circular box; the reading is given by an indicator moving over a divided circle on the lid of the box. The divisions seen cut round the circumference show 1⁄100ths of a millimeter. This ingenious gauge measures upwards of 5 mm.
This necessary and important digression has led me away from the consideration of the achromatic objective, and to which I shall now return.
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.