THE MICROSCOPE. 39 



object-glass, was induced, after the report of Fresnel to the 

 Academy of Sciences, to resume them, and in 1827 he brought 

 to this country and to Paris a horizontal microscope, in which 

 the object-glass was composed of three lenses superposed, each 

 having a focus of six lines and a large aperture. This micro- 

 scope had also extra eye-pieces, by which the magnifying 

 power could be increased. A microscope constructed on 

 Amici's plan, by Chevalier, during the stay of that philo- 

 sopher in Paris, was exhibited at the Louvre, and a silver 

 medal was awarded to its maker. 



"Whilst these practical investigations were in progress," 

 says Mr. Koss,* "the subject of achromatism engaged the 

 attention of some of the most profound mathematicians in 

 England." Sir John HerscheU, Professors Airy and Barlow, 

 Mr. Coddington, and others, contributed largely to the theo- 

 retical examination of the subject, and, though the resiolts of 

 their labours were not immediately applicable to the micro- 

 scope, they essentially promoted its improvement. 



For several years prior to 1829, the subject had occupied 

 the mind of a gentleman who, not entirely practical Hke the 

 first, nor purely mathematical Hke the last-mentioned class of 

 observers, was led to the discovery of certain properties in an 

 achromatic combination, which had been before unobserved. 

 These were afterwards experimentally verified; and in the 

 year 1829, a paper on the subject, by the discoverer, Joseph 

 Jackson Lister, Esq., was read to and published by the E.oyal 

 Society. The principles and results thus obtained enabled 

 Mr. Lister to form a combination of lenses, capable of trans- 

 mitting a pencil of fifty degrees with a large field correct in 

 every part. This paper, which was the ground-work of aU 

 the great improvements that have been efiected in this 

 coimtry in the achromatic object-glasses, has tended to raise 

 the compound microscope from its primitive and almost useless 

 condition to that of being the most important instrument ever 

 yet bestowed by art upon the investigator of natm:e, and has 

 gained for the discoverer a lasting reputation. As the results 

 arrived at by Mr. Lister are indispensable to all who would 

 * Art. , Microscope, Penny Cyclopcedia. 



