252 Correspondence of J. Nickles. 
draws of the draw-tube to measure with different objectives from 
“000025” to 00000167” under an amplification of 1700 to 3600 
times; precautions being taken against any disturbance of said 
value, that might arise from the thin glass covering the objec 
Powell and Lealand cobweb micrometer was occasionally 
used, but mostly for corroboration. The Jackson micrometer, 
upon the whole, was found to be more manageable and satisfac 
tory, the shortness of its lines permitting their coincidence with 
the strise to be more readily observed. The light employed was 
sunlight, admitted through a small aperture into a darkened 
room: with no other light could the strie of the highest marked 
frustules be distinctly resolved under an amplification sufficient 
for counting. In order to diminish as much as le the 
chances of error, the counting always embraced as many cot 
tiguous striae as covered a space equal, at least, to that betwee 
the centres of the five-lines of the micrometer, and when practr 
cable a larger number was counted. 
n conclusion, we may remark that our experiments are col 
firmatory of the generally received opinion that stnx closet 
than about 85 in 001” have not yet been resolved. V ether 
this limit is interposed by the physical properties of light, ot 
whether it arises from defects still existing in our apparently 
faultless instruments, remains to be determined. 
Columbus, Ohio, January 18th, 1859. 
1 emia 
Arr, XXXI.—Correspondence of Prof, Jerome Nickles, dated Par 
January 3rd, 1859. 
. 
On the nature of simple bodies—The Comptes sea said Mears 
through a great number of different states, and still arrives a te 
element. ‘While rendering justice to the zeal and patience? taj, 
pretz, we have to regret that these good qualities have been her 
