Microscopes of Powell, Ross, and Smith. By E. M. Nelson. 435 



and ' Smith's quarters ' were long in repute. Some variations too 

 have been since made in the construction in which I have had no 

 part ; but for all, the principle of the two aplanatic foci has furnished 

 the clue." 



Andrew Koss began making Microscope object-glasses in 1832 ; 

 the following list gives a tabulated history of his work. One of his 



Fig. 117. 



most important discoveries was that of the aberration caused by the 

 cover-glass (1837), and its method of correction by lens distancing was 

 suggested to him by Mr. Lister's paper on the two aplanatic foci. In 

 1849 Ross added a correctional collar to the Gillett's condenser ; he was 

 also the inventor of the silver side reflector in 1836. 



In 1855 Mr. Wenham made the correcting collar of objectives in 

 such a manner that it moved the back lenses of the combination 



Fig. 119. 



instead of the front ; this constituted a real advance, and this plan has 

 now become general ; it is mentioned here because Boss made lenses 

 from formulae supplied by Mr. Wenham. In 1850 Ross made a 

 chromatic condenser consisting of three lenses ; the front was a hemi- 

 spherical meniscus, the middle a piano, and the back a crossed lens ; 

 this lens was designed by the Rev. W. Kingsley, professor of mathematics 

 at Cambridge (fig. 118). It was a very good condenser for a non- 

 achromatic one, and its performance was not unlike Abbe's three-lens 



