228 Dr. North or the Angle of Aperture in Microscopes. 
Mr. Pritchard* that large apertures by permitting a longer conju- d 
gate focus, may ultimately enable all organized structure to be— 
investigated with 4 inch glasses, has been fulfilled sooner than he 
probably could have anticipated. Objectives of this grade now 
resolve every known test. 
Of the two unavoidable remainders of error, the spherical be- 
comes the most important for farther reduction ; we thus gain not 
only in what is called definition, that is fineness and delicacy of 
lines, but in light and shade, in depth, and even in light. Take 
a 4 inch object-glass of very fine definition, but an aperture no 
larger than reported by Mr. Quekett (between 60° and 70°) and 
one of the Naviculee whose lines it will resolve, though not too 
readily—one with diagonal lines being best for this purpose. 
Adjust the screw collar for au uncovered object, and let the cover 
on the slider be a thick one, or interpose another piece of thin 
glass; better also for the object to be mounted in balsam. Turn 
down the wick of a lamp and remove the light till there is barely 
light enough to see the lines yet only on a part of the valve; 
fixing the eye on a portion of the surface where the lines seem (0 
be hidden by a dark shade, adjust the front lens for the thickness 
of cover, and this shade will disappear; the lines will come out 
ma- 
ceous test, especially if sun-light be employed, in a strong and 
bold manner (though thickened and perhaps highly prismat 
when its deficiency of spherical correction shows them through 
a mist and does not show a perspective definition of general form. 
Among the Diatomacee, the genus Nitschia was established by 
Hassall who yet makes no mention of its most distinctive cha 
acteristic, the prominent keel, which strikingly distinguishes 
from Synedra (Hxilaria ), as is shown by thé Rev. Wm. Smith: 
yet the founder of the genus, though he expressly not 
connection of the two genera, failed to point out this ma 
difference which at once attracts the attention if a first-rate ob- 
jective be used. : 
If angular aperture be wnnecessarily large, we have the disad- 
vantage of a useless or injurious limitation of foeus—even SUP 
posing the working distance to be not too inconvenient. Taking 
the semi-diameter of front lens as radius, the working distance 
the tangent of the inclination of the extreme rays: when be 
inclination, as in large apertures, is small, the tangent varles more 
nearly as the angle, and any error of focus causes the outer i)" 
* Micrographia, Essay on Solar Microscopes, 1839. 
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