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MICROSCOPY. 699 
measured in water and then in very fluid Canada balsam without 
alteration of the adjustment. It may be quite possible that if the 
lens had been readjusted so as to give the best image for immer- 
sion in balsam, a slightly greater angle might have been obtained ; 
but this would not have been a fair way of making a comparison as 
it is not the mode in which the glass would ever be employed in 
actual practice.” By not saying squarely, Jt is probably true that 
if the lens had been readjusted so as to give the best image for im- 
mersion in water, a greater angle would have been obtained; and 
this would have been the fair way of making the measurement, as it 
is the mode in which the glass wauld be employed in actual practice, 
Mr. Brooke lost a rare opportunity to do a noble if not a generous 
act. As he is well known to be incapable of an intentional sophis- 
try which by adroitly worded phrase should suggest a doubt where 
none is felt, belittle the concessions which are called for by mani- 
fest truth, and say one thing which is true but has no relation to 
the case at issue, and at the same time imply another thing which 
does relate to the case but is unqualifiedly incorrect, there is no 
choice but to conclude that his extraordinary statement, notwith- 
standing its tone of judicial coolness, was made without that de- 
liberation which the official character of the address demanded. 
On the other hand a still more recent lens by the same maker, 
claiming still more excessive aperture, has been examined by Mr. 
Wenham by his method of cutting off false light described in the 
August number of the Naturauist. By this method, which would 
seem incapable of excluding any image-forming rays, he succeeded 
in obtaining a clear and distinctly limited angle for the lens whose 
light, when not thus protected, was vague and uncertain; the an- 
gular aperture at the same time being reduced from “‘ 180°” to 
“112°” which corresponded within a few degrees with the aperture 
computed trigonometrically from the width of the front lens and 
the length of the working focus. To this it is answered that with 
a dry object on the cover there is no distance involved and the 
triangle is impracticable ; while accurate focussing upon 4 stop 
Which is feasible at “uncovered” adjustment, is liable to error 
from spherical aberration when adjusted for maximum angle. Mr. 
Tolles’ method of demonstrating the utilization of extra-limital 
Tays is by placing a central stop upon the posterior surface of the 
back system of lenses, so large as to cut off all light when the 
f illumination can the 
