PRACTICAL E.\Er?CISES WITH THE MICROSCOPE 255 



and substitute the Cassegrain in a centering collar, or the 

 Leuchtbild condenser with its centering arrangement. 

 Remove the screens from the source of light. Put immer- 

 sion oil on the condenser. Center these condensers by the 

 already centered image of the source, putting a conspicuous 

 part of the object in the field. The Cassegrain has an 

 engraved ring which helps centering with low powers. 

 Since each has a centering ring, when once centered they 

 can be used again, the centering not being disturbed. For 

 observation, the apochromatic objective 60 of 1.3 aperture 

 with the 20 eyepiece forms a good combination. It should 

 be corrected if necessary, for cover-glass deficiency from 0.17 

 millimeter, and requires about 5 millimeters increase in tube 

 length for a cover-glass 0.10 millimeter thick. (Watson, 

 Zeiss.) 



19. Objects in Air, Water, and Immersion Oil. — Make 

 three slides of stained bacteria, and three slides of Surirella 

 gemma, under cover-glasses 0.17 millimeter thick, mounted 

 on the slide, in air, water, and immersion oil, respectively. 

 Observe all the sUdes with (a) a high dry objective, (b) a 

 water-immersion objective (with collar), and (c) an oil- 

 immersion objective, using a good condenser. Note the 

 cutting down of the cone of the water-immersion and oil- 

 immersion objectives on the objects in air; and of the oil- 

 immersion objective of 1.4 aperture on the objects in water. 

 Observe that the corrections of the dry objective are 

 undisturbed on focusing up or down on objects in air, but 

 not on objects in water or immersion oil; that the oil- 

 immersion and water-immersion objectives show errors on 

 objects in air a few microns below the cover; and that the 

 oil-immersion objective is injured by having to focus 

 through a few microns of water, and the water-immersion 

 objective by having to focus through a few microns of 

 immersion oil. 



20. Effects of Reduction of Aperture in Oil -immersion 

 Objective.— Take an oil-immersion objective fitted with an 

 iris diaphragm, like the apochromatic 60, of 1.0 aperture, of 

 Zeiss. Focus on a specimen of Surirella gemma in hyrax, 



