10 HISTOEY OF THK MICROSCOPE. 



In 1826, Professor Amici, who from the year 1815 to 1824 had 

 abandoned his experiments on the achromatic object-glass, was in- ' 

 duced, after the report of Fresnel to the Academy of Science, to re- 

 sume 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 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. 

 Boss, " the subject of achromatism engaged the attention of some of 

 the most profoimd 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 micro- 

 scope, they essentially promoted its improvement." 



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 improvements in the manufacture of object-glasses, 

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

 'of a transparent cement, Canada balsam. This is desii-able to be taken 

 as a basis for the microscopic object-glass : it diminishes very nearly 

 half the loss of light from reflection, which is considerable at the 

 numerous surfaces of a combination ; the clearness of the field and 

 brightness of the picture is evidently increased by doing this ; and it 

 prevents any dewiness or vegetation from forming on the inner sur- 

 faces. Since this time, Mr. Eoss has been constantly employed in 

 bringing the manufacture of object-glasses to their greatest perfection, 

 and at length they have attained to their present improved manufac- 

 ture. Having applied Mr. Lister's principles with a degree of success 

 never anticipated, so perfect were the corrections given to the achro- 

 matic object-glass, so completely were the errors of sphericity and 

 dispersion balanced or destroyed, that the circumstance of covering 

 the object with a plate of the thinnest glass or talc disturbed the cor- 

 rections, if they had been adapted to an uncovered object, and ren- 

 dered an object-glass which was perfect under one condition sensibly 

 defective under the other. Here was another and unexpected diflSculty 

 to be overcome, but which was finally accomplished ; for in a commu- 

 nication made to the Society of Arts in 1837, Mr. Ross stated, that by 

 separating the anterior lens in the combination from the other two, he 



