TULLE Y'S A CHROMA TICS. 45 



Before proceeding to treat of objectives, the student may 

 be advised to commence with the 2-inch ocular (an A). 

 If he wishes for more than one power, the i-inch ocular 

 (C) is recommended, it being double the power of the A. 

 An orthoscopic C would perhaps be useful. 



Object-glasses or Objectives. The history of the achro- 

 matic objective is a curious one interesting certainly, but 

 it should teach us the serious lesson not to be dogmatic in 

 our assertions. Biot and Wollaston, the latter especially, 

 were wedded to doublets, and they both predicted, on the 

 faith of certain experiments which were then unsuccessful, 

 that the compound microscope would never excel the 

 simple. How far this prediction has been verified most of 

 our readers will know; but it is certain that Wollaston 

 never thought that within fifty years of his prediction the 

 doublet would be a thing of the past, rarely heard of and 

 never seen. 



Light seems to have dawned upon objective construction 

 through the elder Dollond, who employed two different 

 kinds of glass in the construction of his telescopes. 

 Recognising this principle, several foreign opticians made 

 partly corrected glasses as early as 1824, and at the same 

 time Tulley of London produced the first achromatic objec- 

 tive made in England : it was composed of three lenses 

 and possessed an air angle of 18, which he soon after 

 increased to 38 by placing another corrected combination 

 in front of it. 



In the year 1829, Mr. Joseph Jackson Lister, in his cele- 

 brated paper published in the < Philosophical Transactions 

 of the Royal Society,' pointed out how many of the diffi- 

 culties could be overcome, and exhibited an objective 

 of 50 air angle which gave a large field and a correct 

 image. This advance was so great that it astonished 

 Dr. Goring, who wrote, in his 'Exordium to Microscopic 



