Yorkville enquirer. [volume] (Yorkville, S.C.) 1855-2006, 1910-10-28

great disks as the 6O-inch and 100
inch: the annealing Of the latter re-
quires about 100 days.
The rough disks when successfully
cast and annealed are shipped in very
strong wooden cases cushioned on the
inside. We have never had disk in
jured in shipment
A very important feature in prepar-
ing for the grinding and polishing Of
the glasses is the optical room or
building This must be so constructed
that it can be kept scrupulously free
from dust, and that the temperature
can be under perfect control. Our op-
tical rooms in Pasadena are well pro-
tected from temperature changes; they
can be closed air-tight, and are sup
piled with fresh air at a constant tenn
perature and washed free from dust.
Another important item is the grind
ing and polishing machine. The ma
chimes which l use at present are the
result Of twenty years experience and
gradual Improvement. BY their aid
the glass disks are changed from
rough castings with ragged edges and
irregular surfaces to brilliantly pol
ished optical mirrors, their surfaces
correct in form to within much less
than one one hundredth thousandth
part Of an inch. After this the final
correcting and retouching Of the sur
faces are done mainly by hand, and the
machines plays a less important part
The optical work is divided into four
marked stages as follows: Rough
grinding, fine grinding, polishing, fig
uring and testing, and finally silvering.
The grinding Is done by means Of
large plates or tools" Of cast iron.
the proper curvature. The abrading
material used between the tools and
the glass may be either sand, emery,
or carborundum grains, mixed with an
abundance Of water. Carborundum
grinds most rapidly, because 1t is ex-
tremely hard and sharp. The glass
lies in horizontal position on the
slowly rotating turntable Of the ma
chine, while the grinding plates are
moved about upon its surface, in clip
tical paths or strokes, by the action
Of the machine. Rough grinding Is
comparatively simple, and consists
merely in giving the glass its approx
innate form; this must be done very
deliberately, however, to prevent heat
ing and consequent cracking Of the
glass;
The principal glass of a rejecting
telescope Is fiat on the back and con-
cave on the face. The form Of the
back need be only approximately true,
but the form of the concave face must
be optically perfect. The back is, Of
course, ground with a fiat tool or plate.
and the face with convex one.
Fine grinding is much more difficult.
since at this stage the accurate surface
of revolution of the concave face must
be secured. Every precaution Is now
necessary to insure uniform tempera-
ture conditions and uniform speed of
rotation Of the glass beneath the
grinding plates. Fine-grinding gives
a surface which Is almost mathemati-
cally true, and which is exquisitely fine
and smooth, ready for polishing. This
Is accomplished by using with the iron
grinding plates a succession of finer
and finer grades of carborundum pow
der with water. These powders, Of any
desired degree Of fineness, are easily
prepared by a method Of washing or
elutriating in water.
The tools or plates used for polish
ing are very different from the grind
ing plates; they are much lighter, be-
ing constructed Of basis Of aluminium
or wood, coated on one face with small
squares of moderately soft rosin or
pitch, which in turn is coated with
thin layer of beeswaX. The polishing
powder used is soft red optical rough
mixed with an abundance of clean wa
ter The polishing tools are moved
about upon the glass by the same ma
chine with which the grinding Is done.
At this stage, as we'll as in the fine
grinding, the utmost care and cleanl-
ness are needed to prevent scratches.
If the fine grinding has been well done,
eight or ten hours work Is usually suf-
ficient. in the case Of large glass, to
bring the surface to a perfect bril-
liant polish.
AS soon as the polish begins to appear,
the exact form of the concave surface
can be determined by optical tests. AS
the polishing proceeds, this testing Is
done several times a day, and the
stroke of the polishing tools is modified
so as to produce more and more nearly
the exact form Of surface desired. This
operation is called figuring the sur
face. 1t will be seen that polishing,
figuring, and testing are carried on
simultaneously, and that the produc-
ton of the optically perfect surface Is
accomplished with polishing tools in
stead of with grinding tools. Even the
finest grinding would be too crude for
the purpose; and besides, the surface
must be a polished one in order to be
tested equently.-Harper's Magazine
GLAD SHE WASN'T STOUT.