OP THE MICROSCOPE. 81 



of two different kinds of glass, while the spherical aberration 

 is corrected partly by the form of the lens, but chiefly by re- 

 ducing the aperture, and by properly combining a series of 

 single lenses, which, however, are never especially adjusted to 

 each other, as in the English forms. Each objective, in its 

 most perfect condition, consists of three lenses screwed to- 

 gether, and in the lower powers these lenses may be separated 

 and used either singly or in combinations of two or three. As 

 the magnifying power obtained with two lenses is less than 

 that obtained by three, the defects of the double combination 

 are not as obvious as they would be if the magnifying power 

 were equal to that of the triple combination. As, however, the 

 spherical aberration in the case of a single lens, whether it be a 

 plain lens or an achromatic combination, is always greater than 

 that of a doublet, and the aberration in the doublet greater 

 than that in the triplet, it is never a good plan to attempt to 

 obtain a low or moderate power by separating the lenses of a 

 high power objective, and using them singly or in twos. Any 

 person having a few French objectives at hand who will try this 

 and attempt to secure the same magnifying power by the use of 

 two lenses, and also by the use of three, the latter being a regu- 

 larly adjusted combination, will find that the results obtained 

 by the use of the latter are far superior to those afforded by the 

 former.* 



Considering their quality, these French objectives are re- 

 markably cheap. Thus a French No. 2, which is nearly equiv- 

 alent to the one-fourth objective of the English and American 

 opticians, can be bought for $5, while the cheapest student's 

 objective of this power would cost at least double that sum. In 

 addition to this, the French objective may be divided so as to 

 afford two other objectives of about three-quarters and one 

 inch eacl), and although the performance of these is far inferior 

 to English or American objectives of the same power, they are 



*In making such a trial, it is, of course, necessary to use lenses of equal 

 quality in both cases, since the quality of the professedly achromatic 

 French objectives in market varies very much. We have seen objectives 

 of this class of the same magnifying power, one of which would not resolve 

 ihe markings on the scales of the clothes-moth's wing, while the other 

 would resolve the Pleurosigma BaUicum. 



