128 THE USE OF THE MICROSCOPE 



they do not deteriorate, and when subsequently screwed 

 into their boxes they are quite dry. Under the bell jar, 

 with the microscope and its attached objectives, there may 

 be a wide-mouthed jar of fused calcium chloride. If the 

 edge of the bell jar rests on rubber, and any liquid which col- 

 lects in the jar is often poured out, the drying material in a 

 half-pint jar will last a year; so that the microscope is dried, 

 and kept thoroughly dry, even if used every day. This pre- 

 caution of course is more necessary when the condenser is 

 water immersed. 



Where dry and oil-immersion objectives are made of 

 similar brass cylinders, and so are not readily distinguish- 

 able, it is well to run an India ink line around the oil- 

 immersion objective, and fix it with cellulose lacquer. A 

 single line can be used on an objective of 1.3 aperture, and 

 a double line on an objective of 1.4 aperture. 



Summary. — For magnifications up to 50, the twin- 

 objective binocular is often preferable. Two objectives, 

 a 10 and 40, of 0.2 (to 0.3), and 0.65 apertures respectively, 

 are mostly employed in routine work, with fixed tube 

 length. Only if the use of a correction collar or alteration 

 of tube length is regularly practiced, or if only covers 0.17 

 millimeter thick are used, should the 40 objective of 0.85 

 aperture be employed. Dry objectives of higher apertures 

 than 0.85 are not usually desirable (Conrady), unless with 

 concave fnonts (Zeiss). The 20 objective may replace the 

 40 more often than is realized (Beck). The 50 or 60 oil 

 immersion of about 1.0 aperture may often be used in 

 routine work instead of the 90 or 100 objective of 1.25 

 or 1.3 aperture. The 60 apochromatic of 1.3 aperture is 

 useful with high eyepieces (such as 20 compensating), 

 having several advantages. The 90 achromatic or apochro- 

 matic of 1.3 aperture probably remains generally the best 

 working (observing) lens. Objectives of 1.4 aperture 

 require more careful adjustment, or they may not be 

 equal, for biological work, to those of 1.3 aperture. Apo- 

 chromatic objectives are only slightly better than achro- 

 matic objectives when both are used with green light. With 



