demnation, which for many years it received, it is now 

 generally accepted as a necessary adjunct to a com- 

 plete outfit, should in fact be part of an equipment in 

 which there is a medium power dry or an oil immersion 

 objective. From single lenses, compound and achro- 

 matic lenses, the use of eyepieces and objectives as con- 

 densers and any number of devices for regulating the 

 light, the generally accepted forms at the present time 

 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. 49, and the other with 

 an aperture of 1.42 of three lenses, Fig. 50. A third 

 is made achromatic with an aperture of i.o. It is, 

 hqwever, considerably more expensive. 



Fig. 49. Fig. 50. 



The particularly distinguishing feature of these 

 condensers is that they will transmit a beam of light as 

 large as can pass within the limits of the substage ring. 



The one of 1.20 N. A. is that in most common use 

 as it meets the conditions for all except the most 

 critical requirements. 



The condensers are mounted in a variety of forms 

 offering greater or less facility for vertical adjustment, 



