140 THE USE OF THE MICROSCOPE 



be adjusted by small screws and a screw driver; and can 

 find no difference in accuracy, but the former is more 

 troublesome. Long-focus condensers alone were used, 

 however. (With short-focus condensers, or with some 

 dark-field condensers, like Nelson's Cassegrain, a special 

 centering sleeve is doubtless necessary.) If the objectives 

 have been bought separately from the nosepiece, however, 

 the low powers may not be quite concentric with the high 

 powers. It is, of course, well in this case, as Coles (46) 

 points out, to vary the position of the series of objectives 

 on the nosepiece until the best results are attained, when 

 the position of each is to be marked. Zeiss now makes a 

 centering ring, which can be put on each objective, for 

 especially accurate work. 



The accurate centering of the objectives is so important 

 that a practiced microscopist may sometimes have to do it 

 for himself. In some nosepieces there are laterally centering 

 devices for each objective. In others there are adjust- 

 ments for the whole nosepiece which is made true through- 

 out, and the objective shoulders are also made true. In 

 nosepieces of the latter type, the final adjustment for each 

 objective is done (by the optician), presumably by grinding 

 the meeting surfaces to the correct plane. The micro- 

 scopist can proceed in the following way: first, test the 

 centering, with and without the nosepiece, of a high-power, 

 oil-immersion objective which gives optimum images. 

 If the centers are the same, the nosepiece is correct; if not, 

 the nosepiece must be adjusted by the maker or by the 

 skilled user. (The bearing surfaces of the objectives 

 must be clean.) With a correct nosepiece, all low and 

 medium objectives can be made to register with the high 

 oil-immersion objective by putting a thin film of cellulose 

 varnish or gum arable at the right place on the objective 

 shoulder, and polishing it down to the exact thickness 

 required. 



In the writer's experience, ''parfocal" objectives are 

 not necessary for high-power work, for the high objective 

 is always raised before rotating the nosepiece. 



