132 THE USE OF THE MICROSCOPE 



Audi habe i('h zuweileji den I']in(lnick gchabt, (lass icli die mil, der 

 Zeisschen Wasser-immersion (2.5, n. Ap. = 1.25) erreichte Bildklarheit 

 mit der Oelimmersiou, die ungleich jener keine Korrektions-fassung hat, 

 nicht erreichte. 



The real use of the water-immersion objective, however, 

 is on objects in a medium approximating 1.33 in refractive 

 index. 



Kinds of Water-immersion Objectives. — Omitting the 

 low-power plankton lenses, there are but few kinds of water- 

 immersion objectives now manufactured. The widespread 

 use of the oil-immersion objective, especially in medical 

 schools, where it is unrivaled for the observation of stained 

 bacteria without a cover-glass, has rendered the water- 

 immersion objective (which was in universal use up to 

 1878, when Abbe calculated the first commercial oil-immer- 

 sion objective) nearly obsolete. The microscope firm of 

 Zeiss, however, makes three excellent water-immersion 

 objectives: 



1. The one-sixth of 0.75 aperture, which gives the best 

 definition in the middle field only, and by this sacrifice 

 allows more latitude in cover-glass thickness. This 

 objective was used in the first ultramicroscope by Sieden- 

 topf and Zsigmondy in 1903. Mann perhaps referred to 

 it in Science.^ 



2. The achromatic objective 90, of aperture 1.18, with 

 a correction collar, gives excellent definition, with covers 

 near 0.17 millimeter thick and a 12.5 times eyepiece. 



3. The apochromatic objective 70 (2.5 millimeters), has 

 the maximum aperture of 1.25; and is a fine objective both 

 for observation and photography, when used with a cover 

 between 0.16 and 0.17 millimeter, and a 15 eyepiece. One 

 or two other opticians, as already noted, also make water- 

 immersion objectives. 



Correction Collar. — The high-power, water-immersion 

 objective should not be used without a correction collar, 

 since 0.01 millimeter of cover-glass thickness makes a 



' Science, 55: 539. 1922. 



