160 ON THE CHOICE OF A MICROSCOPE. 
be considered, I on the whole incline to the opinion that the 
greatest number of advantages are secured in Mr. Ross’s 
instrument that are compatible one with another.* 
Thus much with respect to the mechanical department of 
microscopes. I must now say a few words on the optical 
part, and especially that most important part—achromatic 
object-glasses. The-object-glass may in truth be regarded as 
the eye of the microscope; that which is termed the eye- 
glass standing in about the same relation to it in importance 
that a hand magnifier does to the unassisted eyesight. 
As the hand-glass would be useless if we had no eyes, so 
would be the microscope eye-piece but for the object-glass. 
Of what consequence, then, is it that the object-glass should 
be as perfect as possible! It is this perfecting of the achro- 
matic object-glass that has engaged for the last forty years 
so many sagacious heads, mathematicians and opticians in 
England and abroad, and in the final accomplishment of which 
they have justly earned a lasting reputation. For, to quote 
Mr. Quekett’s words in his treatise on the microscope, “ Of 
all the triumphs of science that have been achieved by a combi- 
nation of the labour of the mathematician and the workman, 
no one can outvie, in delicacy of construction and impor- 
tance, a well-made achromatic combination.” Thus, Franen- 
hofer, Selligues, Chevalier, Aimici, on the continent ; 
Herschell, Airy, Barlow, Coddington, Lister, Ross, of our 
own country; and the eminent opticians Messrs. Thomas Ross, 
Powell, and Smith, now resident in London, are all names 
that will be ever gratefully remembered by the lovers of 
microscopic research in their connection more or less directly 
with the achromatic object-glass. And perhaps foremost 
amongst these, for the tangible results at least that they 
have actually worked out and effected, may be placed, without 
any partiality, those of Mr. Joseph Jackson Lister and 
Mr. Andrew Ross. Mr. Lister’s, for his suggestion, that of 
the double convex lens, a, which, with the plano-concave 
A B c 
OU ie 7 | oy ae 
lens, B, go to make up an achromatic combination, the latter 
* Dr. Carpenter strongly recommends either of the three great micro- 
scopes by Messrs. Powell, Smith, and Ross (‘ The Microscope,’ p. 96) as the 
most desirable for any one, who, according to his means and requirements, 
wishes to possess a first-class instrument; but he gives the first place to 
Mr. Ross’, though for reasons not precisely the same as those that have been 
here advanced. : 
