Notice of a new Object Glass. F 31 
* With this machine I employ superfine emery and water. It 
cuts best by a dragging motion; pulling the crank half round 
with one hand whilst the lever is kept down by the other; and 
then lifting up the lever and allowing the crank to go the other 
half round without grinding. By alternately lifting up the lever 
and pressing it down, the emery works into the cut; whereas, i 
the lever be kept steadily down either by a weight or by the 
hand, and the crank turned constantly round, the drill will be 
cut away much faster than the glass. This is not an expensive 
apparatus.* ; 
The same directions apply to ce- 8. 
menting the round and oval cells to ie 
the slides as those already given; 
and when finished they appear like 
fig. 8, where a@ represents the slide, 
6 the cell cemented to it, and ¢ the 
well formed for the reception of the 
preparation and the fluid. 
Art. IV.—WNotice of a new Object Glass made by Charles A, 
Spencer of Canastota, N.Y. ; by Atexanver 8. Jounson, Esq. 
Dear Sir :—In the latter part of July last I had the pleasure 
of visiting Mr. Spencer at Canastota, and of seeing a ;';th inch 
objective which he had then lately completed upon his new for- 
mula. Its qualities and performances so far exceeded those of 
any objective that I have ever seen or heard of, that I cannot 
doubt your microscopic readers will be pleased to be informed of 
them. Its angle of aperture was measured by us, with a brass 
circle graduated to half degrees. The body of the microscope 
Was adjusted upon the circle so as to bring the objective to its 
focal distance from the centre of motion and the objective was 
illuminated by the direct rays of the sun, care of course being 
taken that no reflected rays should fall upon it. We measured 
from the point on one side where the extreme edge of the field 
was dark, the rest being full of light, to the corresponding point 
on the other side. The angle thus given upon repeated trials 
was 17430, | 
The objective was then adjusted upon a Navicula Spencerii, 
one of the smallest individuals of that species, illuminated by 
* Holes can only be drilled in the very thin glass, (that used for eovers,) only by 
cementing a number of them together and facing them with a thicker glass; } 
pe “0A gua any, some years ago, until I suggested this plan, which 
difficulty. : 5 heaps 
ve 
to us by Dr. J. 
[A v Se ee a cee ing circular cells is to take a hick gla ; 
off Tequisite bore aad ha slices sawed of the thickness ee 
has been recommended J. Leidy of Philadelphia—Eps] 0 
