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.
Magnifying Powers of Eye-Pieces and Objectives.
A TYPICAL AND INITIAL SELECTION OF POWERS OF EYE-PIECES CALCULATED FOR THE 10-INCH TUBE-LENGTH.
| Name | A | B | C | D | E | F | ||
|---|---|---|---|---|---|---|---|---|
| of Maker. | 0 or No. 1 | 2 | 3 | 4 | 5 | 6 | ||
| Baker | 6 | 8 | 12 | 15 | — | — | Diameters. | |
| Beck, R. & J. | 4 | 8 | 15 | 20 | 25 | not made. | " | |
| Leitz | 5 | 6 | 7 | 8 | 10 | 12 | " | |
| Powell & Lealand | 5 | 7·5 | 10 | 20 | 40 | " | " | |
| Reichert | 2·5 | 3·5 | 4 | 5 | 6·5 | " | " | [24] |
| Ross | 3 | 8 | 12½ | 20 | 25 | 40 | " | [25] |
| Swift & Son | 6 | 9 | 12 | 15 | 18 | 21 | " | |
| Watson & Sons | 4 | 6 | 8 | 10 | 12 | 15 | " | |
| Zeiss | 3 | 4 | 5·5 | 7 | 9 | not made. | " |
Compensating Eye-pieces for use with Apochromatic Objectives.
| Zeiss | 2 | 4 | 8 | 12 | 18 | 27 | Diameters. |
This may be taken as a typical set, further treated of among Eye-pieces.