150 THE HISTOEY AND DEVELOPMENT OF THE MICROSCOPE 



much light was inevitably stopped out by the small diaphragm that 

 it was needful to use in order to secure a fair image, the objectives 

 used with this instrument gave a vast increase of light by permit- 

 ting the employment of the full aperture. 



An extremely interesting instrument by C. Chevalier, made very 

 probably not long after 1824, and bearing much resemblance to that 

 of Selligue, is shown in fig. 117. It is provided with a revolving 

 disc of diaphragms applied below the dark chamber under the stage, 



and this is a plan which obtained 

 a permanent place in the micro- 

 scopes of the future. 



The report of Fresnel con- 

 cerning Selligue's achromatic- 

 microscope determined Professor 

 Amici, who for nine years had 

 abandoned his experiments on 

 achromatic object-glasses, to re- 

 commence them in 1826, and in 

 1827 he exhibited in Paris and 

 in London a horizontal micro- 

 scope. The real novelty shown 

 in it was the application of a 

 right-angled prism immediately 

 above the objective to deflect 

 the rays through the horizontal 

 body-tube. The object-glasses 

 were composed of three lenses 

 superposed, each having a focus 

 of three lines and a greatly in- 

 creased aperture. It had al^> 

 extra eye-pieces by means of 

 which the amplification could be 

 increased. 



Meantime the subject of 

 achromatism was engaging the 

 attention of the most distin- 

 guished English mathematicians. 

 Sir John Herschel, Sir George 

 (then Professor) Airy, Professor 

 Barlow, Mr. Coddington, and 



several others, worked more or less at the general subject. Cod- 

 dington alone, however, confined his attention to the microscope, 

 and his work w~as limited to the eye-piece. Also, for some years, 

 Joseph J. Lister had been earnestly working experimentally and 

 mathematically on the same subject, and he discovered certain pro- 

 perties in an achromatic combination, which were of importance, 

 although they had not been before observed. 1 In 1829 a p,-ij :er 

 from Lister was received and published by the Royal Society, 2 

 and putting the principles it laid down into practice, Lister was 

 enabled to obtain a combination of lenses capable of transmitting a 

 1 Vide Objectives, ch. v. p. 355. 2 Trans. Boy. Sac. for 1829. 



FIG. 117. C. Chevalier's achromatic 

 microscope (circa 1824). 



