THE PAST AND FUTURE OF THE MICROSCOPE 203 



SO that a dark field can be had with an oil-immersion objec- 

 tive of 1.3 aperture. The low-apertured (histological) 

 oil-immersion objectives, both achromatic and apochroma- 

 tic, made especially to accompany these condensers of 

 lower aperture, are so useful in ordinary microscopical work 

 that their manufacture will probably not be discontinued, 

 even if oil-immersion objectives of higher aperture are 

 used with the dark field. It has been found that resolution 

 with these condensers is often equal to that of a bright field 

 of the same aperture (Beck, Nelson, Merlin) ; but the reason 

 for this is not apparent yet. Theory seemed to postulate 

 a resolution equal to that of the aperture of the objective 

 added to the highest aperture of the condenser and divided 

 by 4 (Siedentopf ) . 



Other Discoveries and Improvements. — Siedentopf's 

 binocular attachment (Bitumi or Bitukni), and similar 

 arrangements by others, have aided the change from 

 monocular to binocular (stereoscopic or non-stereoscopic), 

 which is taking place in instruments for long-continued 

 work. Among other late discoveries may be mentioned 

 the experimental proof by Beck of the advantage of the 

 use of a small source of light, or a diaphragm on the source 

 of light — a principle which has in another form been long 

 upheld by Nelson. This involves the use of corrected 

 condensers, and the largest possible condenser cones — the 

 advantage of which has been fully demonstrated by Nelson 

 (101), Conrady (48, 49), and Beck (32, 33). 



Other improvements are: the extended use of low-power 

 binocular magnifiers after the pattern of the Greenough; 

 the formation of an objective from rings containing the 

 lenses, which are assembled in a tube, instead of being 

 screwed together (Zeiss, Bausch and Lomb) ; the aspheric 

 condenser (Zeiss, Bausch and Lomb) ; the homal projection 

 lenses for photography with a flat field (Boegehold and 

 Koehler) ; and the miniature camera attachment to the 

 microscope employing these honials (Siedentopf). 



Minor improvements of this century are the use of 

 yellow-green glass, or Wratten light filters (Spitta, Mees); 



