SUB-STAGE CONDENSEES 



305 



contact. Its aplanatic aperture is -9 X.A. 1 '0. We illustrate this 



form in fig. 247. 



FIG. -247. Beck's condenser with N.A. TO. 



It is with great pleasure that we are able to announce the 

 production, by the firm of Zeiss, of a ' centi-iit</ oil-immersion achro- 

 matic condenser' of. X.A. 1-30. This is what we have long desired 

 to see. and we have used it with admirable results. Jt gives a large 

 illuminating aplanatic. c-oiie, hence verv oblique illuminatinor ra ys 



rpi , - J 



Hie centring ar- 

 rangement is the 

 same as that of the 

 achromatic conrlen- 

 ><'] of 1 lie same firm 

 living 1-0 X.A. It 

 is supplied with an 

 iris diaphragm of 

 the most perfect 

 workmanship, and 

 the condenser is 

 focussed not only by 

 rack - and - pinion 

 movement, but also 

 ?>>/ means of a special 

 fine adjustment ; this 

 is accomplished 1 >y 

 the aid of a rotating 



ring provided with a differential thread, as will IM- seen \>\ examining 

 the illustration we give in fig. 248. This allows the condenser to 

 be easily focussed ' at intervals of about ()()] mm.' 'By means of 

 this fine adjustment the condenser may lie focussed up to about 

 1 mm.' 



Messrs. Swift and Son make a panachromatic dry condenser 

 having a X.A. 1 (".), an aplanatic cone of (HIM, and it works well when 

 a critical image is desired. It is well corrected for colour; they 

 also make a panaplanatic oil-immersion of X.A. 1'40. with an 

 aplanatic cone of 1'25. The new optical glass is used throughout 

 the system. It is mounted in an adjustable cell, if desired, for 

 correcting the variations in the thickness of the glass slide. The 



x 



FIG. 248. Zeiss's centring oil-immersion arliroinutir 

 condenser (ls;i:i . 



