THE I'AST AND FTTUUE OF THE MICROSCOPE 201 



Condensers. — Among Abbe's niicroHcopiciil inventions 

 are the Abbe camera lucida, the apertometer, and the 

 silver-grating test plate. He also had made, in 1888, 

 for the apochromatic objectives, an achromatic condenser 

 with large lenses, giving a desirably large image; whereas 

 some condensers were (and are) mostly made with small 

 lenses, giving too small an image. (The writer had this 

 condenser in 1895, and has used it since for dry objectives.) 

 But the example of Koch, who used the uncorrected 

 condenser, made the bacteriologists, who probably bought 

 most of the oil-immersion objectives sold, retain the old 

 uncorrected condenser of 1873, used dry, not immersed; 

 though Abbe had designed a better three-lens type for 

 immersion. This latter is apparently the type which 

 gives an aplanatic cone of 0.5 when immersed, according 

 to DalHnger; whereas the writer found that one form of the 

 two-lens type, used dry, gave a fair cone of only about 

 0.3, with a 3-miinmeter source of light. Bacteriologists 

 thus may be using a method of illumination 50 years out 

 of date, and unsuited to the improved modern objectives, 

 however well suited it may have been to the objectives 

 of 1873, which would not bear a nearly full cone. Con- 

 sequently, workers have to employ a lower eyepiece magnifi- 

 cation than the optimum. 



In 1899, Zeiss introduced a centering oil-immersion 

 achromatic condenser (six-lens) with fine adjustment and 

 large lenses, giving an aperture of 1.3. An excellent 

 aplanatic-achromatic (six-lens) oil-immersion condenser 

 with large lenses was also calculated by Metz, and made 

 by Leitz, especially for use in a dark field. The writer 

 has used the latter continually for several years with satis- 

 faction, both for bright and dark field. The Spencer 

 Lens Company, and Watson and Sons, also make immersion 

 achromatic condensers. 



The Greenough Binocular. — In 1897, the firm of Zeiss 

 first manufactured the Greenough, or twin-objective, 

 binocular, with erecting prisms. This is a good stereo- 

 scopic binocular, with maximum aperture of 0.1 (or some- 



