THE OBJECTIVE 129 



yellow-green light, fluorite objectives may sometimes be 

 preferred to apochromatics (Coles). Objectives may be 

 classified, for high-power work, as searcher objectives, 

 viewer objectives, and observing objectives. No higher 

 eyepiece should be regularly used than will cause the total 

 magnification to equal the sum of objective and used 

 condenser apertures multiplied by 500. Causes of over 

 and undercorrection are given. For objects in water, 

 water-immersion objectives are often best. Whatever 

 high-power objective is possessed, if the best is made out 

 of it, it will approach fairly closely to the limit of observa- 

 tion; for the difference between an ordinary 90 or 100 

 achromatic objective of 1.3 aperture, optically clean and 

 properly illuminated and adjusted, and a 90 apochromatic 

 objective of 1.4 aperture (costing three times as much), 

 uncleaned and incorrectly illuminated and adjusted, is 

 usually in favor of the former. 



Practical Points 



1. If necessary, purchase fewer objectives, in order to be able 

 to get a corrected condenser instead of an uncorrected one, for 

 scientific work; as well as a supply of cover-glasses 0.1 7 millimeter 

 thick. 



2. Keep in mind the limits of useful magnification for the 

 objectives in hand, with full condenser cones. 



3. Use apochromatic objectives, if possible, for all but 

 routine work, unless green light is regularly employed. 



4. With achromatic or fluorite objectives employ a yellow- 

 green color screen for difficult research work. 



5. The high dry 40 objective of 0.65 aperture may well be 

 omitted for most work and replaced by a dry 20, or a 50 or 60 

 oil immersion. 



6. For most objects in aqueous fluids, use, if possible, water- 

 immersion objectives. 



7. Use the oil-immersion objective of 1.4 aperture on stained 

 slides with a solid cone from a corrected condenser of 1.2 to 1.3 

 utilized aperture. If this causes glare on well-stained prepara- 

 tions, some adjustment is wrong. 



