THE PAST AM) FUTURE OF THE MICROSCOPE 205 



carelessness in the construction or in the care of the appa- 

 ratus. For small condensers, and especially for dark-field 

 condensers with a narrow range of aperture (such as 

 Nelson's Cassegrain), exact centering of the condenser is 

 needful for the highest objectives. The range of motion 

 needed is (or should be) usually less than a tenth of a 

 millimeter. A separate small centering collar for each such 

 condenser seems practically best, for it may be set once for 

 all for each. It need not have large screw heads. The 

 eccentric device used on the Zeiss cardioid of 1925 is simple 

 and good. It must be remembered that there is usually 

 only one highest objective on the nosepiece for which the 

 centering of the condenser should be correct to a few 

 hundredths of a millimeter, while in practice the centering 

 is often done with a low objective. A professional micro- 

 scopist cannot afford to waste time on a large centering 

 substage, with large loose screws having a possible range 

 of several millimeters. The writer has had such a substage 

 for years, and it required testing continually before begin- 

 ning work. The accuracy of the maker can usually be 

 trusted for centering, and, in fact, the centrality of the 

 iris diaphragm with the condenser lenses appears to be 

 taken on trust by all. A large neglected centering substage 

 with screws wrong is worse than useless, and in the press of 

 work these screws are likely to be neglected. The writer 

 prefers, for example, to the large centering substage, the 

 centered sleeve of Zeiss, with the tightening screw, on 

 which two water-immersion condensers of other makers 

 (Bausch and Lomb, and Leitz) center well, and have been 

 used daily for several years. There are three adjusting 

 screws turned by a screwdriver. 



4. Traversing and Rotating Iris. — Another arrangement 

 which appears useless to the writer (except for the testing 

 of objectives, which can however be done without such 

 apparatus) is the traversing and rotating movements of the 

 iris, still retained in the Abbe illuminating apparatus. 

 Oblique light, which was fashionable when Abbe published 

 his account of this apparatus in 1873, has been now replaced 



