Dr. North on the Angle of Aperture in Microscopes. 227 
_. The following experiment is interesting : 
Let any one who has an objective with a large angle, try it in 
aroom nearly dark, no light being had except from a small lam 
oracandle in an adjoining one. Let the rays from the distant 
candle shine on the stage of the microscope, which yet receives 
so little light that with the naked eye the smaller letters cannot 
be read upon an engraved bank bill which is laid on the stage, 
and receives no condensation of light from a lens or a concave 
Large aperture being so important, it has become customary 
simply to mention its extent in commending an objective. Dr. 
Goring seldom mentioned the magnifying power of an object 
glass needed for a particular test; but in regard to the tests 
definition sufficient for counting with a micrometer, is the only 
standard which can enable one investigator to compare his obser- 
Vatious with those of another. 
f late, the superior correction which opticians can accom plish 
on medium powers, makes them more efficient than higher ones 
even on tests for which the greater expansion of image of the 
latter is an important advantage. A smaller angle now bares 
accomplish more than one larger by a half. Ihe auticipation o 
scopists felt, on reading the report of the Jury of the Great Exhibition, pronouncing 
the objectives of M. Nachet te he uncorrected for spherical aberration. But this ar- 
tist’s angles fall much chort of Mr. Spencer's. Persons accustomed to the effect of 
'efined spherical correction were less surprised. 
