Brashear, John A. "A Practical Method of Working Rock Salt Surfaces for Optical Purposes."
The Sidereal Messenger 5.5 (1886): 149-151.

Keeler, J. E. "Note on Repolishing Surfaces of Rock-Salt."
The Sidereal Messenger 5.7 (1886): 222-223.

The Sidereal Messenger
Vol. 5, No. 5
May 1886

pg. 149

A Practical Method of Working Rock Salt Surfaces for Optical Purposes.*
Jno. A. Brashear, Pittsburg.

 Two years or more since Prof. LANGLEY asked me to under-
take the work of polishing the rock salt trains so frequently
called into use in his well-known researches in obscure heat
rays. These surfaces, under the very best conditions, are ephemeral in their character,

owing to the deliquescent nature of the
material, and in order to get the best results from them, frequent

repolishing and refiguring are absolutely necessary. I
have been informed that the French opticians polish all rock
salt surfaces upon broadcloth; and indeed, almost all surfaces
I have tested show them to have been finished upon some yielding material,

as the edges are almost always rounded, or as I
would call it, over corrected. This is fatal to good results in
any optical surface. Mr. GEORGE CLARK of ALVAN CLARK &
SONS polished a prism for Prof. LANGLEY which turned out to
be beautiful in polish and figure. His method was to use a
pitch polisher with "diamantine" (a fine variety of Vienna
lime) as the polishing material. A strong brine was used in-
stead of water, in the ordinary way. Mr. CLARK informed me
that the one great difficulty he met with was to wipe the prism
surfaces after the polishing was completed, he using the arm or
palm of the hand, in preference to anything else. On a few
occasions I have succeeded in this way, but where success may
be had
once, failure may result twenty times, for if any moisture is left on the surface

of the prism or lens, even for a moment of time, it is ruined.
 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

*Read before the American Association for the Advancement of Science, Ann
Arbor Meeting, August, 1885.

pg. 150

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 up-
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 on ordinary
counter sink bit, say 1/4 in. diameter, and at intervals of half an
inch over the entire surface. This is done to relieve 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 a fine sponge and equally distributed
over its surface
. The previously ground and fined salt surface
this work is done the same as in glass working) is now placed
upon the polisher and motion instantly set up in diametral
strokes. I usually walk around the polisher while working a
surface. It is well to note that the motion must be constant, for
a moment's rest is fatal to good results, for the reason that the
surface is quickly eaten away, and irregularly so, owing to the
holes that are in the pitch bed. Now comes the most important part

of this method. After a few minutes' work the moisture will begin to

evaporate quite rapidly. No new application of water is to be made,

but a careful watch must be kept upon the pitch bed and as the last vestige of

moisture disappears the prism is to be slipped off the polisher in a perfectly
horizontal direction, and if the work has been well done, a
clean, bright and dry surface is the result. The surface is now
tested by the well known method of interference from a perfect
glass test plate.

pg. 151

 If an error of concavity presents itself the process of polishing is gone over again,

using short diametral strokes. If the error is one of convexity, the polishing

strokes are to be made along the chords, extending over the edge of the polisher.

The essential feature of this method is the fact that the surface is
wiped dry in the final strokes, thus getting rid of the one great
difficulty of pitch polishing, a method undoubtedly far superior
to that of polishing on broadcloth. If in the final strokes the
surface is not quite cleaned I usually breathe upon the pitch
bed, and thus by condensation place enough moisture upon it to
give a few more strokes, finishing just the same as before. In
ten minutes I have polished prisms of rock salt in this manner
that have not only shown the D line double, but Prof. LANGLEY
has informed me that his assistant, Mr. KEELER (J. E.), has
seen the nickel line clearly between the D lines, as well as every
line that can be seen by the use of a good flint glass prism, al-
the dispersion is not so great.
This speaks for the superiority of the surfaces

over those polished on broadcloth.
 In polishing prisms I prefer to work them on top of the polisher as they can be

easily held, but as it is difficult to hold lenses or planes in this way, without injuring the surfaces, I
usually support them in a block of soft wood, turned so as to
touch only at their edges, and work the polisher over them.
Though it takes considerable practice to succeed at first, the results are so good,

that it well repays the few hours' work it requires to master the few difficulties it presents.

Brashear, John A. "A Practical Method of Working Rock Salt Surfaces for Optical Purposes."
The Sidereal Messenger 5.5 (1886): 149-151.

The Sidereal Messenger
Vol. 5, No. 7
July, 1886

pg. 222

Proceedings of the American Association for August 1885, Mr.
J. A. BRASHEAR describes a method for producing optical surfaces of rock-salt

which in his hands gives results leaving little
to be desired in the way of accuracy of figure and brilliancy of
polish. These surfaces, however, when exposed to the air, deteriorate more

or less rapidly, according to the hygrometric
conditions at the time, and soon become opaque and unfit for
use. As the change in an ordinary dry state of the atmosphere
is a gradual one, it is often a rather nice question to decide
when it has advanced to such a point as to render the surfaces
unfit for their work. The production of brilliant surfaces by
Mr. BRASHEAR's method requires, besides the necessary appliances,

much experience in such manipulations, doubtless well
worth acquiring, but not always in the possession of the experimenter,

and thus the restoration of the original polish when
dimmed will in general be beyond his power.
It very frequently happens, however, in investigations in
radiant heat, where rock-salt finds its chief use in physical re-
search, that the extreme accuracy of figure obtained by the
above process is by no means necessary, whereas great transparency, i.e.,

with good material, brilliancy of polish of the
surfaces is always absolutely essential. Under these conditions it is convenient

to have some rough and ready way of

renewing the polish of the dimmed surface, even at the expense of its perfection of figure.
 After considerable experimenting with different substances,
I have found nothing which gives better results for this purpose than thick, soft Canton flannel.

It should be spread out on a smooth flat surface like a table top, or, better, a marble slab,
with the furry side up. Breathe evenly over the surface of the
rock-salt prism or plate and rub it quickly with circular and
then with straight strokes upon the flannel. As soon as it
glides easily, without much friction, remove it and examine

pg. 223

the surface, and if not bright repeat the operation, using a
different part of the cloth. It is best to wear a pair of kid
gloves, to avoid the condensation of moisture from the hands.
A surface almost as bright as the original can be produced in
this way, and the irregularities of figure caused by the rubbing
are surprisingly small. A prism by Mr. BRASHEAR, which had
been treated as many as eight times in this manner by the
writer, still defined the Frauenhofer lines with considerable
sharpness, and the refracting angle had been altered less than
1'. The surfaces, when examined with a test plane by means
of interference bands in sodium light, showed curious irregularities

and a general slight convexity of figure.
 Lenses may be treated in the same manner, the cloth being
held in the hand, unless a rounded surface of approximately
the curvature of the lens is at command, but as they are usually then

they must be handled with great caution. It will of
course be understood that nothing can be done in this way
with surfaces originally bad, nor can much improvement be
made in surfaces which have been badly corroded by exposure
in too moist an atmosphere. The process applies only to surfaces

of good figure which have become dimmed by exposure
under ordinary conditions. After being repolished in this way
as many times as experience shows is allowable, the surfaces
must be treated by a more perfect process like Mr. BRASHEAR'S
in order to restore their original accuracy of figure.


Brashear, John A. "A Practical Method of Working Rock Salt Surfaces for Optical Purposes."
The Sidereal Messenger 5.5 (1886): 149-151.