230 M. Chevalier's Answer to Dr Goring's 



" to correct this defect entirely, since we cannot achromatise 

 lenses as small as those which the microscope requires. 1 ' * 



About this epoch the celebrated Bavarian optician, Fraun- 

 hofer, constructed achromatic lenses having long foci, and con- 

 sequently they were inefficacious upon difficult objects; and 

 they could doubtless be applied only for the observation of 

 objects of large size, which is not generally the purpose of a 

 microscope. 



At the end of 1823, Vincent and Charles Chevalier, opti- 

 cians in Paris, began, on the recommendation of M. Selligue, 

 to construct an achromatic microscope, whose object-glass was 

 composed of four achromatic compound lenses of two lenses, 

 each of the compound lenses having a focus of eighteen lines, 

 with a diameter of six lines, and a thickness at the centre of 

 two lines. These object-glasses were placed, the one before 

 the other, in order that they might be employed, together or 

 separately, in the manner long practised with simple lenses. 

 The aperture placed between these lenses was one line. This 

 microscope was presented to the Academy of Sciences of Paris 

 on the 5th April 1824. M. Fresnel made a favourable re- 

 port upon it on the 30th of August following. As M. Sel- 

 ligue did not inform M. Fresnel that these opticians had taken 

 an active part in the construction of this instrument, and that 

 they had made sacrifices of all kinds for its success, they 

 abandoned M. Selligue and his imperfect microscope, (see Mi- 

 Lister's Memoir,) in which, however, he gave the happy idea 

 of combining achromatic object-glasses, — a method generally 

 adopted, and to which is owing its actual superiority. 



MM. Chevalier and Son were, from that time, occupied 

 alone in the improvement of the object-glasses and of the mi- 

 croscopes; and in September 1824, they had constructed the 



* This natural philosopher does not mention the attempts made by 

 Dollond, Pollard, and several other amateurs, or English and French op- 

 ticians, nor the four object-glasses with three lenses, which, having be- 

 longed to M. Charles, are in the Physical Cabinet of the Conservatory of 

 Aits in Paris. The same may be said of the similar works undertaken in 

 1815 by M. Amid, because the object-glasses then made gave unsatisfac- 

 tory results, and were consequently useless, having only a nominal proper- 

 ty — Note ofM. Chcvalkr. (See Remark K at the End.) 



