158 THE USE OF THE MICROSCOPE 



meters and slides 1.0 millimeter thick. Objectives; apo- 

 chromatic 10 (and 20), achromatic 90 water-immersion 

 (or oil-immersion of 1.0 or less aperture), and 100 fluor- 

 ite oil immersion. Compensating eyepieces, 12.5. (This 

 microscope may also be used for uncovered objects with 

 metallurgical objectives in short mounts.) 



(Z)) Microscope for High-power Apochroinatic Objectives. — 

 Same as (C), but (instead of achromatic objectives) high- 

 power apochromatic objective, 60 of 1.3 aperture or 60 

 of 1.4 aperture, with 20 eyepieces; or 70 water immersion 

 of 1.25 aperture (or 60 oil-immersion of 1.0 aperture), 

 and 90 oil-immersion of 1.3 (or 1.4) aperture, with 15 

 eyepieces. (This microscope may also be used for uncov- 

 ered objects, with objectives specially corrected, and in 

 short mounts.) It may be suggested, for optimum work, 

 that, especially for objectives in short mounts, a triple 

 nosepiece with three of Zeiss's objective centering rings 

 (15 millimeters long) will allow of accurate centering. 



Special Preferences. — The above are combinations of 

 apparatus which may not yet wholly exist. Certain 

 objectives, etc., favored by certain research workers may be 

 noted here ; though, in some cases, they may but have made 

 the best of what chance put in their hands. Thus Coles, 

 for instance, preferred a 1.6-millimeter fluorite objective 

 to a 2-millimeter apochromatic, for observing and photo- 

 graphing spirochsetes and other minute blood parasites. 

 He used yellow-green screens. An American research 

 worker on minute insect chromosomes used a high dry 

 apochromatic objective with a correction collar in examin- 

 ing his balsam mounts of sections, and so avoided hav- 

 ing to clean his covers every time he changed from the 

 finder to the high-apertured objective. Another American 

 worker, who specializes on chromosomes of vertebrates, 

 used a particularly good }yi2 Bausch and Lomb achromatic 

 oil-immersion objective; and employed a low compensating 

 ocular magnifying only twice, for searching with this 

 objective, instead of using a low-power dry objective. 

 The writer of this book avoided soiling his covers by using 



