THE MICROSCOPE. 319 



concave flint-glass one, whose curvature is such that its prismatic 

 power equals that of the crown-glass lens, then, because the prismatic 

 dispersion originating in the concave flint-glass lens counteracts that of 

 the convex crown-glass lens in consequence of the opposition of their 

 curvatures, the former annuls the latter. The image formed by the 

 combined lenses, though its enlargement falls much short of that which 

 the crown-glass lens alone would give, is on the other hand nearly col- 

 orless — achromatic. Not wholly so: from causes whose exposition here 

 would lead us too far, there always remains a colored bordering ; but 

 it is scarcely observable, and for practical uses no longer embarrassing. 



It is generally received that Frauenhofer was the first who (about 

 the year 1811) adopted for microscopes this important improvement, 

 which had long before been applied to astronomical telescopes. The 

 Dutch, who assert for their countrymen the origination of so many inven- 

 tions, claim also for one of them the honor of this, as well as of gunpowder 

 and printing ; and here, it would seem, with better right than in the 

 case of Laurenz Kosur. It is credibly stated that about the end of the 

 last century, Beedsnijder, an optician of Amsterdam, had prepared 

 object-glasses of this kind of pretty good quality ; Van Deyl very good 

 ones about 1807. 



Something was thus gained, but not a great deal. The Frauenhofer 

 object-glasses gave no very considerable enlargement. The spherical 

 aberration was still present, and necessitated the use of a narrow open- 

 ing. The idea of obviating the aberration by the combination of sev- 

 eral lenses, selected with a view to the counteraction of their respective 

 faults one by another, was first carried into execution by Selligue, in 

 1824. This measure was of the most decided advantage. The spher- 

 ical, and in great part the remainder of the chromatic aberration, could 

 be now conveniently corrected, inasmuch as the distances between the 

 successive, and in themselves nearly achromatic lenses could be experi- 

 mentally adjusted, until the image cast by them should be infected 

 with the fewest possible faults. The practical opticians pressed forward 

 with zeal in the newly-opened path. Before all, Amici, in Florence; 

 next to him the English opticians, Ross, Smith, and Beck; followed 

 by the Dutch, Plossl, Schieck, Merz, the French Chevalier, Ober- 

 hauser, offered and offers in this way instruments of high perfection, 

 and far excelling in every respect the single microscope, with a faculty 

 of magnifying those of Amici to the extent of 500, and the Dutch about 

 300 times, and with an unimpeachable clearness and sharpness. By 

 further approximation of the object to the object-glasses, and lengthen- 

 ing the tube of the instrument, as well as by the employment of more 

 strongly magnifying eye-glasses, the size of the image indeed may be 

 increased, but not its distinctness. We see no longer, in the more en- 

 larged image, lines and points as before. 



These are the approximate limits of the working capacity of our 

 present microscopes. An enlargement of more than 800 times can in 

 no case be employed with advantage. 



Opposed to the high figures which itinerant microscope exhibitors 

 give out as the magnifying capacity of their instruments, the low ones 

 we have stated will surprise many readers. In explanation, a few 



