SCIOPTIC ON MANUAL.
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last fifty years, and still made by Duboscq, and other
French manufacturers, would be the best form ; and, as re¬
gards the equal distribution of light on the screen this is
true, but when such lenses are thus used and of sufficient
size to secure this result, their errors of spherical aberration
and want of flatness become unendurable.
We are then fenced in on either side by the necessity of
a large and short lens to secure an equal illumination, and
the difficulty in securing flatness or correction under these
conditions.
The most successful compromise which we have yet found
in this connection is the gas microscope objective, of 11-inch
focus, made by Mr. J. Zentmayer, the well known manufac¬
turer of microscopic stands and lenses.
With one of these, a well defined object, such as a lady-
bug, mosquito, or the like, may be thrown on the screen
with a clear image, pretty well defined up to the margin,
and a field of light so brilliant and regular that it is hardly
distinguishable from that of an ordinary magic lantern pro¬
jecting a colored glass slide of the same object. Of course,
with such a power, very minute objects must be rejected,
but by a judicious selection a large series of interesting ones
can be secured, such as the lady bug or mosquito already
mentioned, the ant lion, field spider, and various water
insects or larvæ of mosquitoes, and the different sorts of
flies ; also wood sections, and even objects so small as the
eye of a dragon fly, but, above all, with this power may be
most successfully shown what are by far the most popular
illustrations with the gas microscope, such living specimens
as the various larvæ above mentioned, and such other
things as are to be found in stagnant water. For these tin-
very simple and effective form of life slide, devised by Mr.
S. Holman, Actuary of the Franklin Institute, Philadelphia,
is invaluable.