Year: 1873

Making Telescope Glasses.


In Professor Newcomb's article on the
new telescope at Washington in Scrib-
ner's for November occurs this accoun,
of the process employed by the manu-
facturers, Messrs. Alvan Clark & Sons,
in grinding, polishing and perfeeting the
immense object 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 appearance of huge,
shallow saucers, Three of these tools
were necessary; one nearly fiat, for the
inner surface of the flint glass, one con-
vex, for its outer surface, and one con-
cave, for the crown glass. The surface
of tie 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 sue
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while, at the same time the operators
continually move around IN the other
direction, so that the strokes are made
successfully in every direction on the
tool. The result of these combined mo-
tions is that every inequality, either on
the glass or on the tool, Is gradually
H.. L 81
tions of nearly perfect spheres. When
this result is attained, the grinding is
continued with emery of continually
greater degrees of fneness, until the sur
face becomes quite smooth.


The next operation Is that of polish
ing. The whole tool is covered with a
thin coating of pitch, which is pressed
while still warm into the proper shape.
1t is then covered with layer of water
and the polishing rouge, and the glass is
again laid upon it, and kept in motion
in the same way as in the fine grinding.
Thus each surface of the two glasses is
speedily brought to a high polish.


The operations we have thus far de-
ecribed require no extraordinary skill
on the part of the workman. With a
little patience and practice almost any
one can make himself better glass than
any known before the invention of the
achromatic telescope. But, such a glass
would by no means serve the purpose OF
astronomy now, and the chances of a
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 rectify the figure. Two for
midable difficulties have to be overcome,
to find what the defects of the glass are,
and where they are situated, and then to
/ remove them without introducing others
To find the defects, the glasses are put
together, set upon edge, facing a lumin-
ous point at a distance equal to ten OF
fifteen times the focal length. The i'm
age of the point formed in 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 oi
the glass noted as the light from the
point passes through it. By these means
1 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 tho
TOOL and the polishing process is recon-
menced, only pressing upon those parts
Of the glass where it has to be ground
away. The glass is then tired again
and again returned to the polisher. The
defects in small glass can be thus pol
ished away in few days, but as the size
/ is increased the process becomes more
and more tedious, and the difficulties Of
/ judging what the defects are, and Of
handling the glass on the polisher so as
diminish them, increase enormously. Il
is this tentative process of gradually
working out every defect of figure, and
even in compensating defects in the uni
formity of the glass itself by suitable
change of figure, that the Clarks have
exhibited their unrivalled skill.