110 



PHOTOMICROGRAPHY 



secured by the use of a film of oil between the condenser 

 and the under side of the object- slip, permitting a 

 ray COK to pass right through to the object without 

 deflection. In the absence of oil a similar ray shown 

 to the left of AB is totally reflected at D, and the most 

 oblique ray utilised is represented by EFG at a con- 

 siderably smaller angle to the perpendicular. The highest 

 possible N.A. that can be used with a dry condenser 

 is 1-0, while oil-immersion condensers are made with a 

 N.A. of 1-3 or 1-4 and an y aplanatic aperture nearly or 

 quite as large. 



Leitz oil-immersion condenser illustrated at Fig. 36 

 has a N.A. of 1-4, and by removal of one or more of the 

 component lenses can be used as a medium- or low-power 

 condenser as follows : 



Apochromatic Condensers (Fig. 37). Messrs. Swift, 

 and Powell and Lealand, call the finest condensers they 



Fig. 37 



SWIFT'S APOCHROMATIC CONDENSER 



make " Apochromatic," as, by the employment of 

 fluorite in their construction, they claim to achieve the 

 corrections corresponding to that term. The front lens 

 should never be removed jfrom these as they are very 



