ROBERT'S TEST PLATES. 
a power of 450 will render the lines of the tenth band equally 
visible; indeed, it is not necessary at all to have recourse to the 
microscope to ascertain the effect which a given magnifying power 
ought to produce upon a band of a given degree of closeness, since 
it is evident that the effect must be merely to make the lines com¬ 
posing the bands more widely separated than they are in the exact 
proportion of the magnifying power. Thus, if the lines composing 
a band, separated by intervals of the 10000th part of an inch, be 
viewed with the magnifying power of 100, they will appear as 
those of a band separated by intervals of the 100th of an inch ; and 
if it be viewed with a magnifying power of 1000, it will appear as 
if the lines were separated by the 10th of an inch, and so on. 
Row, let us apply this obvious principle to the case given in the 
report of the Juries; a magnifying power of 100 directed upon the 
first band, would make the lines appear as if they were separated 
by intervals of the 112th part of an inch; those of the second 
band would appear separated by intervals of the 131st part of an 
inch, and those of the third by the 153rd part of an inch. Row, 
all these would, as admitted in the report, be distinctly seen as 
separate lines, by eyes of average power. But let us see what 
effect a magnifying power of 2000 would produce upon the closest 
of the bands. 
Since it would render the apparent intervals between line and 
line 2000 times greater than they are, those between the lines of 
the tenth band, would be the 25th; those of the ninth, the 19th; 
and those of the eighth, the 17th part of an inch. 
Although it must be quite evident that such intervals are much 
greater than is necessary to enable any eye whatever that can see 
at all, to perceive the lines distinctly separated, the reader will 
be enabled better to appreciate the point by referring to the 
numbers which we have placed on the right of fig. 24, which 
express severally the number of lines to an inch in each of the 
bands composing that figure; thus, the lines of the bands B and c 
are separated by intervals of the 48th part of an inch ; and it 
follows, therefore, that a magnifying power directed upon the band ,• 
x of the test-plate, mentioned in the report of the Juries, would, 
if viewed by a power of 2000, show the lines separated by intervals 
twice as great, or equal to those of every other line in the bands 
B and c, fig. 24. 
For these reasons, it appears to me that a mistake has been t 
committed in the report of the Juries in this point, and I have 
thought it the more desirable to call attention to it, inasmuch 
as the statement has been reproduced in several recent works 
upon the microscope. 
It is easy to show what would be the degree of closeness of the 
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