obtained from the chromatic and spherical corrections. 

 The objective as well as the eyepiece is always com- 

 posed of a series of lenses, the purpose of which will 

 be explained as we proceed. The advent of achro- 

 matic lenses was the first decisive step in advance and 

 has been the foundation of all latter improvements and 

 the high standard of the best production of the present 

 day. 



It is a matter of pride to Americans to note that 

 two of our countrymen, now deceased, were influential 

 in furthering the progress to a considerable extent, and 

 their memories should always be honored by the micro- 

 scopical world. They should be remembered with 

 feelings of gratitude, particularly as the compensation 

 for their efforts was extremely limited. The pioneer in 

 microscopical optics in this country was Charles E. 

 Spencer who was followed by Robert B. Tolles, and 

 while both men did a great amount of original advanced 

 work, it is the latter, particularly, who, by his wonderful 

 achievements, created a great discussion in European 

 circles, by obtaining results which for a long time it 

 was claimed could not be accomplished. 



Of inestimable value to the scientific world have 

 been the labors of that most capable and genial gentle- 

 man, 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, camera lucida, apertometer and apochromatic 



56 



