The following table gives the numerical apertures of objectives of the various systems and of various angular apertures. In the dry system n., representing the index of refraction of air is taken as 1.00; in the immersion systems n equals 1.33 for water, 1.52 for cedar oil and 1.66 for monobromnaphthalin. A glance at the table will suffice to show the great advantage which the immersion objectives have over those of the dry series.
| Angular aperture 2 u | 10° | 20° | 30° | 40° | 50° | 60° | 70° | 80° | 90° | 100° | 110° | 120° | 130° | 140° |
Numerical apertures.
| Dry series n = 1,00 | 0,09 | 0,18 | 0,26 | 0,34 | 0,42 | 0,50 | 0,57 | 0,64 | 0,71 | 0,77 | 0,82 | 0,87 | 0,91 | 0,94 |
| Water-Immersion n = 1,33 | 0,12 | 0,24 | 0,35 | 0,46 | 0,56 | 0,66 | 0,76 | 0,85 | 0,94 | 1,02 | 1,09 | 1,15 | 1,20 | 1,25 |
| Homogeneous Oil-Immersion n = 1,52 | 0,14 | 0,26 | 0,40 | 0,52 | 0,64 | 0,76 | 0,87 | 0,98 | 1,07 | 1,16 | 1,24 | 1,32 | 1,38 | 1,43 |
| Monobromide of Naphthaline-Immersion n = 1,66 | 0,15 | 0,29 | 0,43 | 0,57 | 0,70 | 0,83 | 0,95 | 1,07 | 1,17 | 1,27 | 1,36 | 1,44 | 1,50 | 1,56 |
Photographic objectives with iris diaphragms, f/3.5
| Focal length mm | 64 | 42 | 35 | 24 |
| Price $ | 20 | 18 | 18 | 18 |
Objective 1b.
It is designed principally for drawing extended sections and large specimens, and consists of two adjustable doublets. By varying their distance, a changeable magnification down to two diameters may be obtained.