50 



Ross they are placed at the back or more distant 

 point in the instrument, thus increasing- by means 

 of the connecting arm the faults which might exist 

 in the adjustment. 



A certain form of instrument which at the 

 present is very popular and called the Continental 

 pattern, from the fact that it was made originally 

 by the manufacturers on the Continent of Europe, 

 is a combination of both the Jackson and Ross 

 models. Whereas the coarse adjustment, when 

 consisting of a rack and pinion, is placed closely to 

 the tubes, the fine adjustment is placed in the arm. 



There is another direction, however, in which 

 microscopes are' divided into two classes, which is 

 of far more importance, and affects their utility in 

 a much higher degree. The writer does not know 

 that instruments have been so classified by others, 

 but knows that they can be with perfect propriety. 

 The distance between the eyepiece and the objec- 

 tive is one of vital importance in the optical results 

 and as there are several lengths of tube, the optical 

 qualities of an objective are injuriously affected 

 if it is used with a different length than that for 

 which it was originally intended. It is therefore 

 proposed to classify microscopes according to their 

 tube length as long tube and slwrt ////;r instruments. 



Tube Length. In the Continental form (Fig. 

 20), a short tube from 160.0 to 170.0 mm. (6.3 to 6.7 

 inches) is used, whereas in the English form, this 



