Tue MICROSCOPE. 159 
giving such a visibility to the diatom and at the same time util- 
izing the full aperture of the objective, can hardly be imagined 
by one who has never seenit. The dots upon Amphipleura 
pellucida are shown in a way which would readily convince 
those who still deny their existence. Even Rhizosolmia alata, 
yields transverse lines, which, so far as I know, have never been 
seen by any other method. 
The Vertical Iuminator needs the most careful manipula- 
tion in order to obtain its best results. A small, flat-wicked 
lamp should be used, and placed at such a distance from the ap- 
erture in the mounting of the I]luminator, that it will come toa 
correct focus upon the object when the objective is properly cor- 
rected. The aperture should be left wide open until this condi- 
tion has been obtained, and then the diaphragm may be so turned 
as to allow a narrow marginal beam to enter, which will strike 
the object obliquely. Then the stage should be rotated or slide 
turned until the light strikes the object in a direction perpen- 
dicular to its lines. In case it be desired to show the dots, the 
light may impinge diagonally upon the lines, or two beams of 
light may be admitted. 
HOW TO HARDEN BALSAM MOUNTS. 
“T wish to learn what is considered the best way to harden 
balsam mountings for the microscope, quickly and safely; both 
the pure and that in chloroform, etc. How long should it take 
with either kind, and what would be the approximate temper- 
ature, and best way of maintaining that temperature?” 
In reply to the above we can only say that we have never 
tried to hasten the drying or hardening of the balsam. Should 
we desire to have a mount become hard quickly, we would use 
balsam of such a consistence that it was fluid only when warm 
and quite solid and firm when cold; or as our correspondent 
suggests we could expose the mounted preparation to alow tem- 
perature ; this could be accomplished by placing the mount zn 
a drying oven orinasand bath. Nearly all specimens, however, 
can be mounted in warm balsam without fear of injury, and 
then as soon as the balsam becomes cool it is firm and hard. 
