124 THE USE OF THE MICROSCOPE 



tives are liable to be dropped. Also, it is important that 

 the objectives should be centered and made to register 

 by the manufacturer, on the nosepiece in use. 



Of the apochromatic objectives, which are usually used 

 for research in high-power work, perhaps 10 and 20 are 

 important in the dry series. The latter, which can be 

 dispensed with for most work, but which saves some 

 changing of eyepieces, is necessary for Coles's method of 

 viewing bacteria, cocci, spirochsetes, and malarial parasites, 

 stained and dry on a dark field. The 90 oil objective of 

 1.3, and the 90 of 1.4 aperture, are also useful for high- 

 power research; unless they are replaced by the 60's of 

 the same apertures, which are often more convenient, 

 because of their longer working distances. If much photog- 

 raphy is done, the 120 apochromatic objective is useful. 

 For microscopists who work with objects in aqueous media, 

 and who have acquired the skill to use a correction collar 

 readily, frequent employment of the apochromatic water- 

 immersion 70 may often repay the extra trouble of measur- 

 ing cover-glasses. Instead of high-power apochromatics, 

 fluorite objectives, used with green light, will often suffice, 

 even for difficult work. 



To get the most out of any of these objectives there 

 must be a proper condenser. 



1. An uncorrected, two- or three-lens condenser, used 

 immersed when necessary, with an illuminated ground 

 glass near its anterior focal plane is often used for routine 

 work. 



2. A dry achromatic condenser is certainly better for 

 high powers. 



3. An aspherically corrected (aplanatic) immersion con- 

 denser, with a deep yellow-green screen, may be used, 

 especially with achromatic objectives. 



4. An aplanatic achromatic immersion condenser is 

 satisfactory for use with apochromatic objectives. If one 

 wishes to economize, it is better to have one objective less, 

 and in its place a corrected condenser; the latter costing 

 only as much as, or in some cases even less than, a high 



