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chromatic and spherical corrections and their 

 qualities which will appear as we proceed. The 

 advent of achromatic lenses was the first decisive 

 step in advance and has been the foundation of all 

 later improvements and the high standard of the 

 best productions of the present day. 



It is a matter of pride to Americans to note that 

 two of our countrymen, now deceased, were influ- 

 ential in furthering- the progress to a considerable 

 extent and that their memories should always be 

 honored by the microscopical world. They should 

 be remembered with feelings of gratitude, particu- 

 larly as the compensation for their efforts was 

 extremely limited. The pioneer in microscopical 

 optics in this country was C. E. Spencer, who was 

 followed by R. B. Tolles, and while both men did 

 a great amount of original advance work, it is the 

 latter particularly who, by his wonderful achieve- 

 ments, created a great discussion in European 

 circles, by enabling results to be obtained which for 

 a long time it was claimed could not be accom- 

 plished. 



Of inestimable value to the scientific world have 

 been the labors of that most capable and genial 

 gentleman, Prof. E. Abbe of Jena, to whom, while 

 best known to the general microscopist for some of 

 his more insignificant improvements, such as the 

 Abbe condenser and Abbe camera lucida, we are 

 much more indebted for his profound disclosure of 



