n wipe dry with a third or fourth cloth; but if the
surfaces all need cleaning I know of no better method than that of
taking the objective out of its cell, always using abundance of soap
and water, and keep in a good humor."

Sec. 69. The Preparation of Flat Surfaces of Rock Salt.

The preliminary grinding is accomplished as in the case of glass,
except that it goes on vastly faster. The polishing process is the
only part of the operation which presents any difficulty. The
following is an extract from a paper on the subject, by Mr. J. A.
Brashear, Pittsburg, Pa, U.S.A, from the Proceedings of the American
Association for the Advancement of Science, 1885. Practically the
same method was shown me by Mr. Cook some years earlier, so that I can
endorse all that Mr. Brashear says, with the following exceptions. We
consider that for small salt surfaces the pitch is better scored into
squares than provided with the holes recommended by Mr. Brashear.

Mr. Brashear's instructions are as follows. After alluding to the
difficulty of drying the polished salt surface--which is of course
wet--Mr. Brashear says:-

"Happily I have no trouble in this respect now, and as my method is
easily carried out by any physicist who desires to work with rock salt
surfaces, it gives me pleasure to explain it. For polishing a prism I
make an ordinary pitch bed of about two and one-half or three times
the area of the surface of the prism to be polished. While the pitch
is still warm I press upon it any approximately flat surface, such as
a piece of ordinary plate glass. The pitch bed is then cooled by a
stream of water, and conical holes are then drilled in the pitch with
an ordinary counter sink bit, say one-quarter of an inch in diameter,
and at intervals of half an inch over the entire surface. This is
done to relieve the atmospheric pressure in the final work. The upper
surface of the pitch is now very slightly warmed and a true plane
surface (usually a glass one, prepared by grinding and polishing three
surfaces in the ordinary way, previously wetted) is pressed upon it
until the pitch surface becomes an approximately true plane itself.
Fortunately, moderately hard pitch retains its figure quite
persistently through short periods and small changes of temperature,
and it always pays to spend a little time in the preparation of the
pitch bed.

"The polisher being now ready, a very small quantity of rouge and
water is taken upon