Paper on Aphmatic Object-Glasses. °/29 



In replying to your critique, I find myself under the neces- 

 sity of making known the different improvements made in mi- 

 croscopes since their origin, more especially as to what con- 

 cerns achromatic lenses ; I shall therefore endeavour to trace a 

 short notice concerning them, in which I shall enlarge particu- 

 larly on what has been done in France in this part of dioptricks. 

 In general, the invention of the compound microscope is attri- 

 buted to Drebbel in 1621 ; there are also some opinions in 

 favour of Zacharias Jansens or Joannidas de Middlebourg, 

 and Fontana. Hook improved it by the addition of several 

 lenses. Huygens investigated its theory as well as Toricelli, 

 who perfected the optical instruments then known. 



Since their time many of the most celebrated geometers and 

 artists have occupied themselves with the improvement of mi- 

 croscopes, and have principally sought to construct an instru- 

 ment which, endowed with considerable magnifying power, 

 light, and clearness, should also be of easy and universal ap- 

 plication. 



Schmitz, Cuff, Euler, Dollond, Adams, Delabarre, Amici, 

 and Charles, much improved compound microscopes ; for two 

 centuries they were constructed in England, in Holland, in 

 Italy, and in France, until this part of the science of dioptricks 

 seemed to be exhausted. Nevertheless new experiments demon- 

 strated that these instruments were still far from perfection ; 

 in fact, an achromatic combination had not been employed in 

 the composition of object-glasses, which is nevertheless an in- 

 dispensable condition for the attainment of perfect images. 



Euler first occupied himself in the perfection of this instru- 

 ment. To him it in fact belongs to have applied to micro- 

 scopes the happy combination of Dollond. (See remark I.) 

 Although achromatic lenses were invented in 17G0, yet it was 

 not till 1774 that Euler proposed to employ them in micro- 

 scopes, and so slow and difficult is the progress of the; arts, 

 that it was not till the end of 182.'} that achromatic lenses for 

 the microscopes were actually constructed. Even in 1821) M. 

 Biot remarks in his Traitc dc J'hi/siijiic, " that u great part 

 of the imperfections of microscopes depends on a want of achro- 

 matism, which lu tomes the more insupportable when we wish 

 to use high powers. It is unfortunate)) impossible) 11 he adds, 



