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Account of a Compound Achromatic Microscope. 237 
Arr. XXVIL—An Account of a Compound Achromatic Micro- 
scope, made by Charles A. Spencer of Canastota, N. Y. ; by 
C. R. Gitman, M.D., Professor of Obstetrics, &c., in the Col- 
lege of Physicians and Surgeons, New York. 
THE great importance of the microscope to the cultivators of 
natural science, and the difficulties and delays which have at- 
tended procuring good instruments from Europe, will, I hope, 
justify me in occupying a small space in this Journal, with an 
account of a microscope which has recently been constructed for 
me, by Mr. Charles A. Spencer of Canastota, N. Y., a gentleman, 
who by force of natural talent, and with no other aid than could 
be derived from the reading of books on optics and the construe- 
tion of the microscope, has attained the highest rank as a prac- 
tical optician. 
‘The microscope is constructed on the model of the smaller mi- 
croscopes of Charles Chevalier, and like them can be used either 
in the horizontal, inclined, or vertical position: it can also be 
inverted so that the stage is directly over the object. 
It differs as to the details of its construction from the instru- 
ments of Chevalier, in being supported on a tripod like Pritch- 
ard’s, instead of being screwed into the top of the box, having a 
safety stage and also a peculiar arrangement by which the object 
can be moved in azimuth. It has a fine polarizing apparatus and 
a Lieberkuhn. 
ing power. . 
A peculiarity of all the objectives, and one on which their ex- 
cellence, as Mr. Spencer thinks, mainly depends, is the large an- 
gle of aperture; this in the one-seventh inch glass is one-half 
greater than in the corresponding power of Chevalier, and in the 
one-twelfth it is nearly double. a 
Those even moderately familiar with the microscope, wi 
readily appreciate the immense advantage which this wide angle 
of aperture gives the glasses as a working instrument. 
The size of the field is great, the three hundred and fifty power 
has a field more than double that of the three hundred and 
twenty-five power of Chevalier. stv 
Of the penetrating and defining power, it Is perhaps not neces- 
sary that I should speak, when the excellence of the instrument 
