HISTORY OP THE MICEOSCOPE. 13 



to be taken as a basis for the microscopic object-glass ; it 

 diminishes very nearly half the loss of light from reflec- 

 tion, 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 surfaces. 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 

 manufacture. Having applied Mr. Lister's principles 

 with a degree of success never anticipated, so perfect 

 were the corrections given to the achromatic 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 corrections, if they had been 

 adapted to an uncovered object, and rendered 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 communication 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 had been completely successful. The 

 construction of this object-glass will be illustrated and 

 explained in a subsequent chapter. 



The rapid improvement of the achromatic micro- 

 scope was greatly furthered by the spirit of liberality 

 evinced by the late Sir David Brewster, Dr. Goring, 

 Messrs. R. H. Solly, Bowerbank, and Wenham. To 

 Dr. Goring we are indebted for the first triplet achro- 

 matic object-glass, for the diamond lens, and for the ^ 

 improved reflecting instrument of Amici by Cuthbert. 



The instruments manufactured by the leading 

 London makers, Messrs. Ross, Powell and Lealand, and 

 Smith and Beck, are unsurpassed in any part of the 

 world. 



American opticians have shown themselves quite 

 equal to their brethren of the old country. The im- , 

 proved forms of instruments manufactured by Tolles, "" 



