234 
MEMORANDA. 
^Treatise on the Microscope/ and Lardner's ^Mnsenm of 
Science and Art/ vol. vi^ p. 69^, and^ most probably^ elsewhere. 
I believe^ however^ that many^ to whom the subject is new, 
look upon these calculations with " the doubts of imperfect 
faith/^ or_, if they accept these measures at alF^ (to use the 
language of the Astronomer Royal, when speaking of very 
different measures, in his most instructive work, the ^ Ipswich 
Lectures on Astronomy^), "they adopt them only upon loose 
personal credit." Under this impression, it has often oc- 
curred to the writer, that it wo aid be both interesting and 
instructive to endeavour to draw the several groups of lines 
by the aid of the camera lucida, at the same time taking the 
^ Jq ^th of an inch from the micrometer. This would afford, as 
it were, an ocular and graphic confirmation of Mr. De la Rue^s 
calculations, sufficient to enable any one to satisfy himself of 
their general truth by mere inspection. This the writer has 
endeavoured to do, as shown in the accompanying diagram ; 
and, though the method can only be considered rather rough 
and approximative, yet he apprehends that the risk of any 
serious error creeping in is only very small, and the table 
below shows that it is sufficiently exact to give very close 
approximations to Mr. De la Rue's results. The test-plate, 
which was used on this occasion, contains fifteen series of lines, 
and appears to be similar to the one mentioned by Quekett 
in his work on the ' Microscope,^ p. 477, ed. 2. I have, 
however, only been able to transfer the first thirteen series, 
and in each series, in the drawing from which this was 
taken, the lines are represented as actually drawn with the 
camera, and, in order to present to the eye its relation to 
y^L^^th of an inch, each series is most carefully continued by 
means of scale and compasses, so as to fill up the interval of 
O'OOl inch, such continuation being drawn in lines only half 
the length of those which represent the lines as dotted down 
from the test-plate. By this means, the approximate interval, 
which separates the lines of any series, can be obtained by 
mere inspection, the fraction of a divisfon being taken by 
estimation. Thus, in the series No. 7, there are 28 intervals 
and about half an interval, or 28*5 intervals in 0' 001 inch ; 
consequently, in one inch, there are 28,500 such intervals, 
and the lines are distant from one another -ssioo^^ 
inch. The results thus obtained are given in this table, side 
by side with Mr. De la Rues^ calculations, and show as close 
an agreement as, I think, could possibly be expected by this 
method. The magnifying power in the first ten series is 
rather more than 1700 diameters, and in the last three above 
2000, both estimated by the usual standard, ten inches. 
