SUJKSTAGE CONDENSEBS 



305 



contact. Its aplanatic aperture is -9 X.A. 1-0. We illustrate thi< 

 form in fig. 247. 



FIG. 247. Beck's condenser with N.A. 1-0. 



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

 production, by the firm of Zeiss, of a ' centring oil-immersion achro- 

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

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

 illuminating aplanatic cone, hence very oblique illuminating rays. 

 The centring ar- 

 rangement is the 

 same as that of the 

 achromatic conden- 

 ser of the same firm 

 having 1-0 N.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 

 by means of a special 

 fine adjustment ; this FIG. 248. Zeiss's centring oil-immersion achromatic 

 is accomplished by condenser (1899). 



the aid of a rotating 



ring provided with a differential thread, as w T ill be seen by examining 

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

 be easily focussed * at intervals of about O'Ol mm.' ' By means of 

 this fine adjustment the condenser may be focussed up to about 

 1 mm.' 



Messrs. Swift and Son make a panachromatic dry condenser 

 having a N.A. I'O, an aplanatic cone of O93, and it works well when 

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

 also make a panaplanatic oil-immersion of N.A. T40, 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 



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