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AMERICAN NATURALIST. 
Vol. VI.— APRIL, 1872.— No. 4. 
EGT: TIMO 
THE USE OF AMPHIPLEURA PELLUCIDA AS A 
; TEST-OBJECT FOR HIGH POWERS. 
BY DR. J. J. WOODWARD. U. S. A. 
Over a year ago (February 1, 1871) the Surgeon General of the 
United States Army published a brief memorandum prepared by 
me, on the Amphipleura pellucida and its markings. This mem- 
orandum was accompanied by two photographs exhibiting the 
striæ of the diatom, as seen with a power of about one thousand 
diameters. The paper was republished in the ‘* American Journal 
of Science and Arts” (May, 1871), and in the ‘London Monthly 
Microscopical Journal ” (July 1871). : 
$ ince preparing the memorandum referred to, I have had occa 
Sion to use the Amphipleura pellucida a number of times as one 
of the means of comparing the high power objectives of various 
makers, and having found it, within certain limits, well adapted to 
this purpose, have thought the following remarks on its use would 
not be without interest to working microscopists. 
pecimens mounted by various English preparers may readily 
be obtained from any of the large dealers in microscopical prep- 
arations, I have compared such modern slides with some of the 
original ones mounted by Messrs. Sollitt and Harrison, which I 
“we to the courtesy of Mr. W. S. Sullivant of Columbus, Ohio, with 
the Sample in the first century of Eulenstein, and with other slides 
a various sources. I find all very much alike, the striæ usu- 
y varying from ninety to one hundred to the zoo Of an inch. 
ha few large frustules I have found coarser striæ than the above 
enang according to Act of Congress, in the year 1872, by the PEABODY ACADEMY. OF 
in the Office of the Librarian of Congress, at Washington. 
AMER. NATURALIST, VOL. VI 13 - (193) 
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