10 HISTORY OF THE 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. 

 Ross, " 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 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, 

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

 of a transparent cement, Canada balsam. This is desirable 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. Ross 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 difficulty 

 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 



