New national era. [volume] (Washington, D.C.) 1870-1874, 1873-12-25
How Telescope Len-r, are aIadr


from Scribnrr's M"tuL,
The tools are very simple round plates al
cast iron, about three feet in diameter, hol-
lowed out TO suit the curves Of the lens They
have somewhat the appearance Of huge ,hal
low sauccrs, or, more nearly STILL Of the ,hdl
low iron drinking cups attached Ia pumps ~
hydrants in some Of our cities. Thee Ot
these tools are ncces7ary-Onr nearly Hat,
Tor the inner surface Of the II nj class; one
convex, for tI outer surface, aud one con
cave, for the crown glass. The surface of the
tool i. covered with coarse emery aud water
the Z lass is Laid upon il, and the grinding ~
carried on by sliding the gla1- back nui forth
on the tool.
While sliding the glass Is slowly turned
round, so that the grinding strokes occur sue
cessiycly in every direction on the glass,
while, at the same time, the operators con
FINALLY move around in the other direction,
so that the strokes are made sGcrr--iycly ia
every direction Oh the tool The result OF
these combined motions Is that every me
quality, either on the gas, OF the tool, IS
gradually worn away, aud both are reduced
10 portions Of nearly perfect spheres. When
this result is attained the grinding IS con
tinueI with emery Of continually greater de-
grees of Hncucss, until the surface become,
quite smooth.
The whole tool i. covered with thin coating
~ pitch, which Is prcs-ch while It'll warm
into the proper shape. II IL then covered
with layer 81 water and the polishing
rouge, aud the giI-1 II again laid upon IL,
and kept in motion IN the same wayas IN the
fine grinding, Tlus each surface 81 the two
glasses is speedily brought t. high polish.
The Operations We have thu tar described
require no extraordinary xii on the part Of
the workman. With "A little patience and
practice almost any one can make himself a
better glass than any known before the III
vention Of the achrOnat.c telescope. But
such glass would<-by ~ means s@tso the
purpose Of astronomy now, and thu chances
Of ylass Of any size tuTn.ng out s&a'tiy
ri2ht on the frst trial are yeas -l2hl. he
skill Of the OptClan Is now called INTO pldJ
u reclify the figure. Two formidable DIN
cullle- have to be overcome, % md what
the defects Of the gla-. are. and where they
are situated and then % remove them with
Oh, Introducing Othcrs. To Ind the deicct-
the glasses were pat together, set P on edge.
!acinga luminous point al - di'tan. - equal
10 10 or f. times the focal length. The
;aa.s Of the psint formed ia the iV-E. OF the
q sss is thep examined with an ele-p.ecc o!
high power, Or the eye j. placed exactly IL
the focrs, aud the aspect Of the @lass noted
us the light from the point passes through II.
BY these mean, the skilled Optician can judge
where ths curves Of the .la.ses aEc too
great, and where too email.
The glasses are now taken back % the
tool asd the polishing process Is recommended
emily pressing upon thOse parts OF the glass
" here Ia has to be ground away. The glas
i. then tired again, and again returned 10 the
poll.her. Tbs defects Ia a small glass caa
be thus polished away III few days, bu, Us
the size Is Increased the process becomes
more and more tedious. aud the difficulties VI
Judging what the delects are, and Of hand
ling the glass Oh the polishef 40 "a to dl-
uulnish them, Increase enormously.