376 SUMMARY OF CURRENT RESEARCHES RELATING TO 



stand. Fig. 97 shows the most primitive form of such a condenser, the 

 whole arrangement resembling an object-slide on whose lower surface a 

 conical mirror K has been cemented by its smaller end. A metal plate 

 B is cemented on to the large end of the frustum k, so as to keep the 

 direct rays back from the preparation. This frustum lies, as shown, in 

 the stage aperture, and as its lower diameter is 14*6 mm., the stage 



i // k- K Y=3 



B 



Fig. 97. 



aperture must be at least 15 mm. in diameter. The axis of the conical 

 mirror is indicated by a diamond scratch on the upper surface of the 

 plate, and by the aid of a weak objective this mark is set in the midst 

 of the field, and the point of the light-cone applied to it by manipulation 

 of the Microscope mirror. The insertion of a strong illuminating lens 

 between the light-source and the Microscope mirror is recommended, as 



x 



' V ' K v 



B 



Fig. 98. 



the illuminated plane of the preparation is thereby much increased. 

 The preparation itself can be laid on the top of the plate without a slide, 

 thereby avoiding the necessity of an immersion. It is obvious that 

 such a method would, however, only be of advantage in a preliminary 

 examination, and would not lend itself to permanent preparations. But 

 the difficulty can be overcome if the permanent preparation (fig. 98) 



X* ' '*;* 



IT* 



\ 



Fig. 99. 



has been mounted on an extremely thin object-slide (about 1 mm. thick). 

 Also the plate condenser must be secured by stage-clamps, and the pre- 

 paration can then be shifted as desired. The application of a drop of 

 immersion fluid is, of course, necessary. 



Fig. 99 represents another very simple arrangement for ultramicro- 

 scopical purposes. The conical mirror is now replaced by a spherically 



