and Powell and Lealand's Immersion y^th.” 159 
inch. In fact, neither assumption is true. The positives were 
copied from the original negatives in an ordinary copying camera. 
When I sent them to Boston, I wrote to Mr. Tolies that they were, 
as compared with the negatives, “ enlarged a little in the copying 
camera.” I also informed him that the frustules represented, 
counted in the microscope, the one 92, the other 93 striae to the 
i-oVoIh of an inch. (This letter was sent July 10th, 1871.) Nothing 
was said then or since of the distance at which the original nega- 
tives were taken, and the powers of these original negatives were 
very different from those of the positive copies. 
Now, the argument of the article referred to is based on three 
suppositions : — 
1. That the positives are of the same size as the original negatives. 
2. That these were each taken by the objectives named, without 
eye-piece, at 48 inches distance. 3. That the frustules represented, 
counted each 95,000 strife to the inch. Each supposition is un- 
true. Perhaps the writer of the article may be able to explain why 
he made these suppositions ; I, for one, would be willing to judge 
his excuses without prejudice. 
Whether now his measurements of the positives are correct, is 
another question, and one which does not affect his argument in the 
least, since the size of the two bears no uniform relation to the mag- 
nifying powers of the two objectives at equal distances. I will only 
say that, as the positives were copied for me by another person, it 
is quite possible that even those from the same negative are not of 
the same size, though I intended they should be. 
Mr. Bicknell’s measures of the two he examined do not corre- 
spond with mine, of my retained copies, but I do not emphasize 
this point because it does not bear on the questions involved. The 
positives were sent out to show the relative defining powers of the 
two objectives, and not to indicate their magnifying powers ; every- 
thing therefore was made subordinate to the first object, and as I 
did not measure each copy, I may perhaps have unintentionally 
fallen into error as to the size of some of these. 
This possibility of erroneous figures as to some of the positives, 
does not apply to my published measurements of the actual magni- 
fying powers of the objective in question ; and as both glasses are 
still in the microscopical collection of the Army Medical Museum, I 
should be happy to give any respectable microscopist an opportunity 
to compare the two objectives as to magnifying power and other 
qualities. 
