570 MICROSCOPY. 
or both plane surfaces fine ground, is all that is necessary, only, 
be it provided, that some part of one edge be a polished or frac- 
tured surface tolerably near flat and square. Use this precisely as 
Dr. Woodward uses his tank, and the angle of the objective for 
that kind of balsam (like the glass) will be indicated along the 
ground surface if a little care be taken in adjusting glass to 
objective. Balsam, glycerine or dense oils will do to connect the 
objective front and glass plate, for the pencil traversing the plate 
will be constantly the same for a wide range of ‘ preservative 
media.” This cone can be marked as to its boundaries with a 
pencil on the ground glass, and measured with a protractor with 
perfect facility. 
hatever position gentlemen respondent may take now, pro 
or con, the end is assured, viz., a practically larger angular aper- 
ture for objects in balsam. I hope you will award these comments 
an insertion. Respectfully yours, 
Rosert B. ToLLES. 
40 Hanover Street, Boston, Mass. 
. S. — Since writing the above, the “ Monthly Microscopical 
Journal” for July, containing Mr. Wenham’s reply to Dr. Wood- 
ward’s article, has come to hand. I notice Mr. Wenham recom- 
mends the same ground glass plate for test of angle that I describe 
above, only nothing is said of connecting media. This is excel- 
lent! With air between, the cone will, with crown or plate glass, 
be about 81°, but if water or balsam or any known liquid replaces 
the air it can be more. It is the test. Some objective will be 
found in England, I dare say, to go above 82°.—T. 
Microscorican Expertments wiru Insects’ Eres.— Dr. F. W. 
Griffin, of the Bristol School of Chemistry, gives in the “ World 
of Science” and in the ‘ Monthly Microscopical Journal,” an mM- 
teresting note on this subject. Any tolerably mounted beetle’s 
eye (transparent) will give some of the desired effects; but for 
good results the semi-globular set of “lenses” which constitutes 
the outer part of the compound eye should be very carefully 
cleaned and flattened without materially altering the form of the 
individual lenses. This is arranged as a transparent object under 
a one inch objective, and preferably a “Kelner” eye-piece, when 
some two thousand lenses or corneules are brought into view at 
once. By racking the objective up, the focus of these little lenses 
