Green-Mountain freeman. [volume] (Montpelier, Vt.) 1844-1884, 1873-10-29
Making Telescope Classes.

In Professor Newcomb's article on the
new telescope at Washington, in Scribner's
tor November, occurs this account OF the
process employed by the manufacturers,
Messrs. AIvan Clark & Sons. in grinding,
polishing and perfectiDg the immense OBJ
ject glasses: The tools are very simple
round plates of cast iron. about three feet
in diameter, followed out to suit the curves
of the lens. They have somewhat the ap-
pearance of huge shallow saucers, OF more
nearly still, of the shallow iron drinking
cups attached to pumps or hydrants in
some of our cities. Three OF these tools
were necessary, one nearly fat, for the in
ner surface of the flnt glass, one convex,
for its outer surface, and ore concave, for
the crown glass. The surface OF the tool
is covered with coarse emery and water,
the glass is laid upon it, and the grinding
is carried on by sliding the glass back and
forth on the tool. While sliding, the
glass is slowly turned round SO that the
grinding strokes occur successfully in
every direction on the glass, while. at the
same time the operators continually move
round in the other direction, so that the
strokes are made successlvely in every d.
recton on the tool. The result of these
combined motions is that every inequality,
either on the glass or tool, is gradually
worn away, and both are reduced to por-
tions Of nearly perfect spheres. When
this result is attained, the grinding is con-
tinued with emery of continually greater
degrees of ineness, until the surface be-
comes quite smooth.
Tae next operation is that of polishing.
The whole tool is covered with thin coat
ing Of pitch, which is pressed while still
warm into the proper shape. 1t is then
covered with a layer of water and the pol
shing rouge, and the glass is then LAID
upon it, aud kept in motion in the same
way as in the fne grinding. Thus each
surface Of the two glasses is speedily
brought to high polish.
The operations we have thus far describ-
ed require no extraordinary skill on the
part of the workman. With little pa
tience and practice, almost any one can
make himself better glass than any
known before the invention of the achro-
matte telescope. But, such glass would
by no means serve the purposes of astron-
omy now, and the chances of glass Of
any size turning out exactly right on the
first trial are very slight. The skill of
the optician is now called into play to rec
tify the figure. Two formidable difficult
ties have to be overcome, to find what the
defects Of the glass are, aud where they
are situated, and then to remove them
without introducing others. To fnd the
defect. the glasses are put together, set
up on edge, facing luminous point at a
distance eq lal to ten or fifteen times the
focal length. The image of the point form
ing the focus of the glass is then examined
with an eye-piece of high power, or the
eye is placed exactly in the focus, and the
aspect of the glass noted as the light from
the point passes through it. By these
means the skilled optician can judge
where the curves of the glasses are too
great, and where too small.
The glasses are now taken back to the
tool and the polishing process is recon-
menc' d, only pressing upon those parts of
the glass where it has to be ground away.
The gass is then tried again, and again
returned to the polisher. The defects in
a small glass can be thus polished away in
few days. but as the SIZE is increased
the process becomes more and more te-
dlous, aud the difficulties of judging W hat
the defects are and of handling the glass
on the pollsher so as to diminish them, in
crease enormously. 1t is in this tentative
process of gradually working out every
defect of figure, and evea In compensating
defects in the uniformity of the glass it.
self by suitable change of figure, that
skill.