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pocket microscopes. In these the reflecting substage 

 mirror is regularly employed. 



Martin, 17 80, shows a good example of the tripod 

 stand. 



Adams, 18 71, introduced diaphragms, and also the 

 button system of combining objectives to increase magnifi- 

 cation. To increase the magnifying power, the objective 

 has an extra lens screwed on. He also used a nose piece 

 for carrying three objectives. 



The next improvement of note was in the quality of 

 objectives. Achromatism was studied, 1759, by Martin, 

 but no really good results eventuated till 182 3, when 

 Chevalier produced Selligues' achromatic objectives. Six 

 years later J. J. Lister read a paper on this subject to 

 the Royal Society, which profoundly inf^.uenced the study 

 of achromatism in England. 



Improvements now began, one after the other, both 

 optical and mechanical, too numerous to mention, but 

 important enough to remind us of such men as Pritchard, 

 Ross, Powell, James Smith, Swift, Watson, Baker, and 

 Professors Quekett and Carpenter, in this country, of 

 Zeiss and Leitz, on the continent. 



America, too, has not been behindhand in adding to 

 constructive excellence. 



1 now come to the wonders revealed by the 

 microscope, and their value to the sum of human 

 knowledge. The field is almost too vast and the ad- 

 vantages too numerous to particularize in every branch. 

 The earliest observers were chiefly concerned with the 

 wonder of seeing flies as large as hens. Poor men, such; 

 as Leeuwenhoek, it is true, ground their own lenses and 

 made their own apparatus, but for many years the 

 microscope was regarded (in the words of Hogg) more in 



