MICROSCOPIC DRAWING AND ENGRAVING. 
I am informed by Mr. De La Rue, that bands engraved upon 
other plates, were observed and computed by himself, Mr. Lister, 
and Mr. Nobert, and the results^ now before me, are in such 
accordance as to leave no doubt of their general accuracy, the 
discrepancy being so trifling as to be explained by the small errors 
inevitable in such observations. 
It will be evident that microscopes, having different degrees 
of power and efficiency, would be necessary to render the lines 
composing the successive bands of such a series distinctly visible; 
to determine what power would be required for each band, it is 
not at all necessary to have recourse to any microscopic observa¬ 
tions; the question simply is, what is the degree of closeness 
of the lines, that the naked eye can barely distinguish as 
separate; this will, of course, be somewhat different for different 
eyes. 
29. The use of these' test-plates in determining the power and 
efficiency of microscopes, will be easily understood; instruments 
of low powers, such, for example, as from 100 to 200, will only 
make the wider bands, such as a b and c, fig. 24, distinctly 
visible, the closer ones, eeg, will be barely visible as dark bands, 
but the lines composing them will not be seen, and the closest of 
the series, iik, will not be seen at all. In proportion as the 
power and efficiency of the microscope is increased, more and 
more of the bands will be visible as distinct series of lines. 
Mr. Nobert supplies test-plates, engraved with bands of diffe¬ 
rent degrees of closeness, according to the power of the instruments 
to which they are to be applied. 
30. In the Report of the Juries of the Great Exhibition of 1851, 
page 268, it is stated, that to see the bands of a test-plate of 10 
bands, such as that described above, a linear magnifying power of 
100 is necessary for the wider bands, such as I and n, but that to 
distinguish those of the closest band, such as x, a magnifying 
power of 2000 is necessary. 
I think it is apparent that this statement is erroneous, being 
evidently incompatible with the relative closeness of the lines of 
the several.bands. Thus, for example, while there are 11265 
lines of the first band to an inch, there arc 49910 lines of the 
tenth band to an inch. Those of the latter are, therefore, only 
4| times closer than those of the former; and it is evident, that 
if these bands be viewed with two microscopes, one having a mag¬ 
nifying power 41 times greater than that of the other, with 
proportional defining and illuminating powers, the lines com¬ 
posing them will appear equally separated; and since it is 
admitted in the report, that a power of 100 will render the lines of 
the first band visible, as it evidently will do, it will follow that 
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