118 THE USE OF THE MICROSCOPE 



magnification, with an aperture of 0.4 to 0.65, will give 

 useful magnifications from 100 to 400, or more; and may be 

 used as a viewer objective for chromosomes, and also as a 

 searcher for spirochetes (Coles, 45, 46). Both these 

 objectives may be corrected for uncovered objects by 

 appropriate increase of tube length; an increase of 35 

 millimeters for the apochromatic 20, according to Coles 

 (46), with 160-millimeter tube-length; and much less for 

 the 10 objective. 



The ^^-inch, or 4.3-millimeter objective (of about 40 

 magnification) is commonly used in routine work. It 

 should not usually have a higher aperture than 0.65, unless 

 a correction collar or alteration of tube length is employed 

 to correct it for different cover-glass thicknesses. The 40 

 achromatic or fluorite dry objective of 0.85 aperture with 

 a correction collar, makes a fair viewer, especially in con- 

 junction with a high-power, water-immersion objective. 

 It is at its best, however, with dry objects only. The 50 

 or 60 oil-immersion objectives, of aperture from 0.85 to 

 1.05 are especially used with dark-field illumination; but they 

 are useful viewers for oil-immersion objectives of higher 

 aperture, and excellent observing objectives for much routine 

 or other work. Their further employment is to be recom- 

 mended, to replace, more or less, the high dry objectives. 



For "observing" (or ''working") objectives of the 

 highest power, experience seems to have fixed on 2 milli- 

 meters (90 magnification) as the best focal length for 

 the apertures of 1.3 and 1.4; though the achromatic K2 

 is usually a ^i^, that is of 1.8-minimeter focus, and 100 

 magnification; and the ifg of 1.5 millimeter focal length 

 is often preferred for photography (Coles, 46, and E. B. 

 Wilson). Abbe (14), however, considered the S-milHmeter 

 objective preferable for the highest powers of the apochro- 

 matics in respect to the definition in the extra-axial parts 

 of the field, and its readier manipulation. It is a comfort- 

 able high power to use with a 20 eyepiece because of its 

 long working distance, and it is often preferred by the 

 writer to objectives of shorter focus, except for photography. 



