Year: 1873

Iu ITOIcSSOT ANcN Cpiio III IielS Oh tlic
new telescope at Washington in Scrib-
ner's for November, occurs this account
of the process employed by the manufac-
turers, Messr5 Alvan Clark & Sons, in
grinding, polishing, and perfecting the
immense object glasses: the tools are We
ry simple, round plates of cast-iron,
about three feet in diameter, followed
out to suit the curve Of the lens. They
have somewhat the appearance of huge
shallow sauc rs, or mo e nearly still,
of the shallow iron drinking cups at
tached to pumps and hydrantS in our
large cities. Three Of these tools were
necessary one nearly flat, for the inner
surfdcee of the flint glass one convex,
for It's outer surface, and one concaye,
for the crown glass. The surface Of the
tool is covered with coarse emery and
water, the glass is laid npon it, and the
grinding is carried on, by sliding the glass
back and forth ON the tool While sli-
ding, the glass is slowly turned round so
that the grinding strokes occur success
lively in every direction on the glass,
while, at the same time the operator
moves around in the other direction, so
that the strokes are made succeSSiyely
in every direction on The tool. The re-
suit OF these combined motions is that
every inequality, on either the glass or
the tool is gradually worn away, and
they reduced to portions ofnearly perfect
spheres. When this result is attained,
tie grinding is continued with emery of
continually greater degrees of hneneSs,
until the surface becomes quite smooth.


The next operation is that of polishing
The whole tool is covered with 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 laid
upon it, and kept in motion i'll the same
way as in the fine grinding. Thus each
surface Of the TWO glasses, is speedily
brought to high polish.


The operations We have thus far des-
cribcd, require no extraordinary skill on
the part of the workman. With little
patience and practice, almost any one
can make himself better glass than
any known be fore the invention Of the
achromatic telescope But such a glass
would by no means serve the purposes
Of astronomy now, and the chances Of
yl- SIS Of any size turning out exactly
right Oh the first trial are very slight.
The skill Of the operative is now called
into play to rectify the figure. Two
formidable difficulties have 20 be over
come; to find what the defects Of the
glass are and then remove them withou,
introducing Others, To find the defectst
the glasses are pat together set up on
edge, facing luminous point at Il dist
ance, equal to ten or fifteen times the
focal length. The image Of the point
cormed in the focus Of Tie glass is then
examined with an eye glass of very high
power, or the eye is placed exactly in
the focus and the aspect Of the glass
noted as the ligh; irom 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 ana the polishing process is re-
commenced, only pressing upon those
parts of the glass that have TO be ground
away. The glass is then tried again,
aud again returned to the polisher; the
defects in a small glass can thus bopol
ishcd away in a few days, but as tie
size is increased. the process becomes
more and more tedious, and the difficult
ties Of indcing what the defects are, and
Of handlnI the glass on the pOilshcr 50
as to reduce them, increase enormously.
1t is this tentative process of gradually
working ont every defect Of figure. and
even in compensating defects in the mi
forlntv Of the glass itself by a suitable
change of figure, that the Clarks have
exhibited their unriyalled skill,


- %----