32 b> Notice of a new Object Glass. © 
reflection of a white cloud from the concave mirror placed not 
obliquely but with its centre in the axis of the microscope. | In 
this way the shell was well resolved into dots. 
At my suggestion a $th objective of French manufacture was 
then used for illumination as an achromatic condenser, with the 
light from a white cloud reflected by the plane mirror, the whole. 
accurately adjusted to the axis of the microscope and the image 
of the cloud brought in focus in the plane of the object, The 
dots upon the shell were thus shown with a sharpness of defini- 
any mode of illumination 
tion that [ have never seen equalled by any other objective with — 
I have a French objective with which by oblique light I can 
see either set of lines on the Navicula Spencerii, but only one 
set atatime. With this objective and oblique light I can resolve — 
all the bands of lines on Nobert’s test of fifteen bands, while 
with the achromatic condenser I can only resolve nine. bands. 
This will serve in some measure to illustrate the comparative 
merits of an object-glass capable of showing the dots on the N. 
Spencerii by oblique light and of one capable of showing them 
by an achromatic condenser. 
We afterwards looked at a Grammatophora subtilissima from — 
Greenport, with the achromatic condenser and the same adjust- 
ment, and I saw the cross lines steadily and with perfect distinct- 
ness, getting at the same time by glimpses the longitudinal ines. 
_ The light failing us, as evening was approaching, we did not — 
try in this way either the Amici test or the Providence Gram- 
matophora, but in the evening we saw both these objects satis- ) 
factorily resolved into dots, by unreflected oblique light from one — 
wick of a common bed-chamber lamp, burning oil, a homely — 
but very effective method of illumination for objectives of large 
aperture. | 
the microscope, by Dr. Wythes of Philadelphia,” in which in the | 
. iy - 
ing the performances of this objective with the recorded perform- | 
ances of other object glasses which from time to time have ap- 
peared in the Journal, I cannot resist the conviction that it is the — 
best objective ever produced, and far the best 
New York, Oct, 9, 1851. 
