150 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



much light was inevitably stopped out by the small diaphragm that 

 it \vas needful to use iu 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 

 prohablv 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 Selligne'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 also 

 extra eye-pieces by means of 

 which the amplification could lie 

 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, Professm 



Fui. 117. C. Che\ < limmatic 



microscope i i-ii ca 1 *.; I . 



Barlow, 31 r. Ooddtngton, and 

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

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



his work was limited to the eyepiece. AKo, for some years, 



eph .). Lister had been earnestly \\orking ex peri mentally and 

 thematically on the same subject .' and he discovered certain pro- 

 perties in an achromatic comhinat ion. which \\< -re of importance, 

 ough they had not been before observed. 1 In I-S29 a paper 

 received and published 1, N the Royal Society, 2 

 <'" principles it laid down into ractice, Lister was 



btain a coml)ina1ion of lense 

 .ch. ,. p. 



capable of transmitting a 

 i Trans, Eoy. Soc.fnv is-jii." 



