50 PEACTICAL PHOTO-MICKOGKAPHY 



The removal of the top combination leads to a lowering 

 of the numerical aperture ; but, as this expedient is only 

 necessary with lenses of medium power, their aperture is corre- 

 spondingly lower, so that no loss results from the alteration. 

 Among the best at present to be obtained is an oil-immersion 

 condenser made by Messrs. J. Swift & Son. It has a total 

 aperture of 1-40, and an aplanatic aperture of 1-30. Its focal 

 length is J inch. With the front hemispherical lens removed 

 it may be used as a dry condenser with a N.A. of 0-62. 

 Another worthy of note is a similar one made by E. Leitz, 

 of the same N.A., but the aplanatic cone nearly equals the 



Fia. 14. Zeiss Centreing-collar for Microscope Objectives 

 used as Condensers. (Full size.) 



total aperture. Both these condensers are capable of fulfil- 

 ling the most exacting requirements. 



The acme of perfection is to use as sub-stage condenser an 

 objective exactly similar to that which is on the microscope ; 

 and Messrs. Zeiss supply a special arrangement for carrying 

 such an objective below the object-stage (Fig. 14). In this it 

 will be seen that the centreing-collar is the same as that in the 

 centreing sub-stage. In place of the condenser, a short sleeve 

 is screwed into a collar, and the latter carries the objective. 

 The illustration shows a 16-mm. apochromat in position. 

 The upper internal screw in the carrier is for short -focus 

 objectives in short mounts, which would otherwise not focus 

 sufficiently closely to the object-stage. While this is an 

 entirely satisfactory method, it is so expensive as to 

 limit very largely its possible use, and the condensers at 

 present to be obtained are so perfect optically that the 



