1901] MICROSCOPICAL JOURNAL. 227 
virtues known. The manufacturers have vied with each 
other, and by the use of the many optical glasses that 
are at their disposal, combined with extensive computa- 
tions and’ experiments and skilful work, they have pro- 
duced lenses of large aperture and fine performance. 
When apochromatic objectives were first introduced their 
performance was in the majority of cases immeasurably 
superior to others. These have been taken as a standard, 
and many lenses at very low cost are being made that 
are ‘practically equal to them excepting only so far as 
the color corrections are concerned. 
at | 3 must not be inferred that this point is neglected, in 
fact it requires a critical eye oftentimes to detect an in- 
feriority _ in this respect, but for the bulk of the work 
undertaken by microscopists it can be disregarded, and the 
cheaper lenses will reveal all that the apochromatics are 
capable of showing. These remarks apply to the prod- 
ucts of nearly every firm of repute, and advice would be 
willingly given by such in the selection of lenses, which 
could be accepted without misgiving. 
_ CONDENSER.—The reason why a condenser having a 
smaller aperture than that possessed by the objective is 
sufficient is because there are no lenses available that 
will bear an illuminating cone equal to their own aper- 
ture. Asa rule definition begins to fail even with the 
best lenses when anything more than a cone of three- 
fourths the numerical aperture of the objective is em- 
ployed. There have been exceptional lenses which have 
borne more than this, but they are rare. It would be 
well worth your while to have’an oil immersion con- 
denser if you were using an objective with a N. A. of 
1 40, , It is possible that this lens would stand a cone 
slightly in excess of 1°0 N. A., and the best dry con- 
densers do not give more than ‘9. Thecone of illumina- 
tion required depends in a large measure on the subjects 
