12 HISTORY OF THE MICROSCOPE. 



purchased by the late Duke of Buckiugham, and another 

 the thirtieth of an inch focus. 



In March 1825, M. Chevalier presented to the Society 

 for the Encouragement of the Sciences an achromatic 

 lens of four lines focus, two lines in diameter, and one line 

 in thickness in the centre. This lens was greatly superior 

 to the one before noticed^ which had been made by him 

 for M. Selligues. 



In 1826, Professor Amici, of Modena, who from the 

 year 1815 to 1824 had abandoned his experiments on the 

 achromatic object-glass, was induced, after the report of 

 Fresnel to the Academy of Science, to resume them; and 

 in 1827 he brought to this country and to Paris a hori- 

 zontal microscope, in which the object-glass was composed 

 of three lenses superposed, each having a focus of six lines 

 and a large aperture. This microscope 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 physician in Paris, was exhibited 

 at the Louvre, and a silver medal was awarded to its 

 maker.' 



" While 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 Herschel, Professors Airy and Barlow, 

 Mr. Coddington, and others, contributed largely to the 

 theoretical examination of the subject; and though the 

 results of their labours were not immediately applicable 

 to the microscope, they essentially promoted its im- 

 provement." 



Mr. Jackson Lister, in 1829, succeeded in forming a 

 combination of lenses upon the theory propounded by 

 these gentlemen, and effected one of the greatest improve- 

 ments in the manufacture of object-glasses, by joining 

 together a plano-concave flint lens and a convex, by means 

 of a transparent cement, Canada balsam. This is desirable 



(1) In 1855, -when the Jury on Microscopes at the Paris Exposition were com- 

 paring the rival instruments, Professor Amici broufjht a compound achromatic 

 microscoi)e. comparatively of small dimensions, -wiiicli exhibited certain striae 

 in test objects better than any of the instruments under cxaminaticm. This 

 superiority was produced by the introduction of a drop of water between the 

 object and the object-glass. 



