436 HISTORY OF LENSES AND MICROSCOPES [Ch. XII 



accomplish achromatism, for if two or more transparent bodies could 

 be combined and neutralize their dispersive effect without overcoming 

 the mean refraction it would be possible to make achromatic combina- 

 tions. This is shown by the course of the beam of white light travers- 

 ing the two prisms (fig. 172). The first to accomplish the feat in a 

 way to make achromatic telescopes possible was John Dollond (1757). 

 Naturally the telescope took the lead in the improvement, as it at that 

 time was by far the most important optical instrument. Furthermore, 

 the lenses were relatively large; for in the differentiation of the tele- 

 scope and microscope the objective of the telescope became pro- 

 gressively larger and that for the microscope progressively smaller. 

 The smaller the lenses the more perfect must be the grinding and 

 polishing, for slight imperfections in their small area introduce 

 obscurations which in the larger surface of the telescope lenses would 

 be negligible (§ 476, fig. 180). But the microscope makers undertook 

 the task in several different countries, — England, France, Russia, 

 Holland, Germany, and Italy — and from 1759 to 1824 were tireless 

 in their efforts. Finally Selligue laid before the French Academy the 

 result of his efforts with the help of the practical opticians, Vincent 

 and Charles Chevalier. From that time on achromatic objectives 

 became more and more common for microscopes, although from their 

 small aperture they were not liked by some workers so w T ell as the 

 more brilliant, uncorrected lenses. 



In our own country, Charles A. Spencer took the lead in trying to 

 overcome the lack of brilliancy in achromatic objectives. He too, 

 early realized and grasped the importance of aperture for the micro- 

 scopic objective. He realized also that for the balancing of the dis- 

 persions and refractions to make true achromatic combinations, it w r as 

 necessary to have materials for lenses with special properties. He 

 worked in tw T o directions. One was the use of the natural mineral 

 fluorite whose properties had been pointed out by Brewster (§ 465a) 

 and the other was the production of new forms of glass with specially 

 desired optical qualities. 



It fills one with admiration to think of this genius with small means 

 working alone in his cramped quarters trying to make new forms of 

 glass, which with the old forms and with natural minerals would enable 



