155 



received, it is now generally accepted as a neces- 

 sary adjunct to a complete outfit, in fact should 

 always accompany an oil immersion objective. 

 From single lenses, compound, non-achromatic 

 and achromatic, the use of eyepieces and objec- 

 tives as condensers with any number of devices 

 for regulating the light, the generally accepted 



Fig. 39. 



Optical part of condenser 1.20 Aperture. 



Fig. 40. 

 Optical part of condenser 1.42 Aperture. 



forms have come to be those devised by Prof. 

 Abbe. One of them with a numerical aperture of 

 1.20 consists of a combination of two lenses (Fig. 

 39) and the other with an aperture of 1.42 of three 

 lenses (Fig. 40). A third is made achromatic with 

 an aperture of 1.0 which, however, is considerably 

 more expensive. 



