152 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



pencil of 50 with a large corrected field. This paper and its 



results exerted a very powerful influence on the immediate improve- 

 ment of English achro- 

 matic object-glasses. MI id 

 formed a per ma unit 

 basis of advancement for 

 the microscope, not only 

 in its optical, but also 

 indirectly in its me- 

 chanical construction 

 and refinements. 



For convenience, at 

 this point we may ad- 

 vance a little in order 

 to complete our brief 

 outline of the mechani- 

 cal application of achro- 

 matism to object-glasses. 

 Mr. A. Ross became 

 practically acquainted 

 with the principles of 

 achromatism as applied 

 to combinations of lei i >< s 

 in working with Pro- 

 t'es.xnr Barlow on this 

 subject, and having ap- 

 plied Lister's principles 

 with great success, he 

 discovered, as we have 

 already pointed out in 

 Ch. I., 1 that by covering 

 the object under exami- 

 nation by a thin film of 

 glass or talc the correc- 

 tions were disturbed if 

 they had been adapted 

 to an uncovered object ; 

 and we have seen that, 

 it was in 1837 that Ross 

 devised a simple means 

 of correcting this. He 

 was an indefatigable 

 worker in the interests 

 of the advancement of 

 the mechanical as well 



FIG. 119. Pritchard's microscope with ' Continental ' as the optical side of the 

 fine adjustment (1835). microscope. Fig. 1 1 8 



presents a form of 



microscope, from an extant example which was designed by W. 



Valentine of Nottingham in March 1831 and made by Andrew Ross. 



1 P. 20. 



