354 OBJECTIVES, EYE-PIECES, THE APERTOMETER 



It has been, however, customary to accredit the first practicable 

 attempts to achromatise object-glasses to M. Selligues. In 1823 

 he suggested to M. Chevalier to superimpose two, three, or 

 four achromatised plano-convex 'doublets,' that is to say, pairs 

 of lenses. These objectives had their convex surfaces presented to 

 the object, which gave them four times as much spherical aberration 

 as would have been the case had their positions been reversed, 1 and, 

 as we have just seen, Marzoli reversed them. This necessitated an 

 excessive reduction of the apertures, which, nevertheless, still too 

 manifestly displayed an obtrusive aberration. Yet the conception 

 of an achromatised combination had been embodied in an initial 

 manner. In 1825 M. Chevalier perceived the exact nature of the 

 mistake made by M. Selligues, and made the lenses of less focal 

 length and more achromatic, and inverted them, placing the plane 

 side of the flint towards the object. 



It is somewhat important, as it is interesting, to note that the 

 idea of the superposition of a combination of lenses did not originate 

 from theoretical considerations of the optical principles involved. 

 It is scarcely conceivable that where there was manifest ignorance 

 of the position of a plano-convex lens for least spherical aberration 

 (a principle now thoroughly understood) there could have been in- 

 sihgt enough either to detect the presence of the two aplanatic foci 

 or to discover a method of balancing them 

 by inductive reasoning. Everything in the 

 history points to happy accident as the primal 

 step in achromatised objectives, and this, with 

 very high probability, applies to the work of 



Chevalier, for Selligues' attempt was a blunder 



PIG. 308.-Tully's achro- a P inst the commonplace knowledge of his 

 matic triple. time. 



The form of three superimposed similar 



achromatic doublets is precisely the combination of the French 

 ' buttons,' which have been sold in thousands until quite recently, 

 many of them being mounted as English objectives. 



At the suggestion of Dr. Goring, Mr. Tully, in this country, 

 without any knowledge of what was being done on the Continent, 

 made an achromatic objective in 1824. This was a single combina- 

 tion, being an achromatic uncemented triplet. It was, in fact, a 

 miniature telescope object-glass, and is illustrated in fig. 308. Two 

 lenses made on this principle by Tully, having T \ and ^ foci, w r ere 

 found in practice too thick, and in many ways imperfect ; and he 

 was induced to make another single triplet of T ^y focus and 18 aper- 

 ture, and its performance was said to be nearly equal to that of 

 the T %. 



Subsequently a doublet was placed in front of a similar triplet of 

 somewhat shorter focus, forming a double combination objective of 

 38 aperture. This was pronounced to be a great advance upon all 

 preceding combinations, even those which had been produced upon 

 the Continent. 



A note of Lister's at this time upon the objectives of Chevalier 



1 Chapter I. 



