20 



manufacturers on the continent of Europe, is a com- 

 bination 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 on the arm and although 

 being dissimilar from the original Ross in being 

 higher, it nevertheless has the disadvantage of mag- 

 nifying any lost motion in the adjustment by means 

 of the connecting arm. Considering the fact that 

 the Ross form alone is almost obsolete and many 

 instruments of the present day are a combination of 

 both forms, it appears to the writer that their desig- 

 nations have lost their value. 



There is another direction, however, in which 

 microscopes are divided into two classes, which is 

 of far more import ance, and affects their utility in 

 a much higher degree. The writer does not know 

 that instruments have been so classified by others, 

 and knows that the subject has been given no 

 important significance. 



In the Continental form just mentioned, a short 

 tube from 160.0 to 170.0 mm. (0.3 to 6.7 inch) is 

 used, whereas in the English form, and this is largely 

 followed in America, the length is from 8| to 10 in. 

 (216.0 to 250.0 mm.). The short tube of the Euro- 

 pean makers offers no optical advantages, but is 

 mainly used to contract the height of the instrument 

 to as great an extent as possible, as this is the vital 

 point throughout its construction. 



