ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



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Fig. 95 illustrates another method of securing a dark field with 

 immersion objectives, the observation-objective being so stopped off that 

 the rays passing by the rim of the disk-diaphragm are detained in the 

 objective. This is effected by the insertion of an intermediate piece 

 introduced between the tube-stop and the objective mount. This 

 intermediate carries a tube-stop C of the required size, corresponding to 

 the aperture of the objective. The stop is made removable so that it 

 does not interfere with the ordinary use of the objective. 



Fig. 96. 



Exchange Condenser. — This is shown in fig. 96, and its construction 

 is due to P. Schmidt. A double-action Abbe condenser of N.A. 1*10 is 

 so combined with a conical condenser that, either the disk-diaphragm B, 

 or the lens L. 2 , together with the iris I, can be inserted. In the first case 

 the condenser functions as an ultra-microscopic illuminating apparatus 

 (as shown in fig. 96) ; in the other case, as an ordinary condenser. 



Plate-Condensers. — Figs. 97-101 illustrate an entirely new class 

 of ultra-microscopical illuminators, and derive their name from the fact 

 that they (especially the simpler forms) bear some resemblance to a 

 glass plate. They have the advantage of being completely independent 

 of the illuminating apparatus of the Microscope, and their application 

 only requires the existence of the Microscope mirror and of a sufficiently 

 large stage-aperture. They could therefore be used with the simplest 



