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Casting of an Optical Glass DiskSource: The Scientific Monthly, Vol. 26, No. 5 (May, 1928), pp. 479-480Published by: American Association for the Advancement of ScienceStable URL: http://www.jstor.org/stable/8017 .
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TEI:IS PROGRESS O}' SCIEN=CFJ 479
THE OPTICAL DISK
CASTING OF AN OPTICAL GLASS DISK TIIE closing act in an interestint, sci-
entific drama that llas lastecl rnore than eit,ht months has been staed at tlle Bu- reau of Standards, Department of Com- merce, +vith the uncoverin^, of the fur- nace containing a great disk of optieal glass, 70 inches in diameter, 11 illches thick and weithing about 3,500 lsounds. WShen the cover +sas reTnoved in the pres- ence of a small but distin^,uished group of scientific experts the glass +^ras found to be practically perfect, and the long period of suspense for those resonsible for its production was orrer. The suc- cessful consummation of this work has resulted from the cooperativc efforts of several men within and outside the bu- -reau, particular credit being due l\fr. J. Walter Drake, former assistant secre- tary of commerce, Dr. S. W. Stratton, -president of the tIassachusetts Institute
of Technolot,y, Professor C. C. Crump, of Ohio Wesleyan IJniversity, and SIr. 1). II. Bates, Dr. A. Q. Tool and SIr. A. :N. Finn, of the Bureau of Standards.
The disk +vill be used as a great con- cave mirror for the nerv reflecting tele- scope of the PerlSins Observatory at Ohio AVesleyan University, Delaware.
The money M ith which to establish this observatory was left to the univer- sity by Professor SIiram Mills Perkins, of Ohio WVesleyan. It +ras his desire to establish an observatory of the first rank at the university and that the entire equipment be of American manufacture. It is significant that Professor Perkins had no desire for the observatory to bear his name, his sole wish being, as he es- llressed it, to enable the students of Ohio Wesleyan and men everywhere to "love God and serve Him more acceptably."
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480 THE SCIENTIFIC MONTHLY
reaehed. It was then annealed at this
Nevertheless, after his death in 1924, his
name was given to the new observatory,
work on whieh had already been in prog-
ress for two years. There was only one real obstaele to
delay ultimate aehievement-large disks
of optieal glass of the size required had
never been made in this eountry and
only twiee abroad. Those that have
been suetessfully eompleted are said to
be imperfeet, and although imperfee-
tions in the glass are not so important
in a refleeting teleseope as in a refraetor,
nevertheless it is always desirable to
have glass as nearly perfeet as possible.
After four unsueeessful attempts to
obtain a disk of the size required, a
unique method was developed by A1r.
A. N. Finn in eharge of the Bureau 's
glass seetion: 1,000 poundls of eullet
(broken glass of the same eomposition as
the glass to be made) and 4,600 pounds
of sand and ehemieals were melted in a
single large pot in a gas-fired furnate.
The molten glass was stirred by hand for
six hours and at the proper time on May
7, 1927, the pot was tapped. The molten
glass was flowed into a mold of the re-
quired size whieh was spetially designed
for this purpose. This mold was at the
same time a earefully insulated anneal-
ing furnaee, provided with eleetrieal
heating elements by means of whieh the
temperature eould be adjusted and eon-
trolled to within a degree. The temperature of the glass when
poured was about 2,400° F. For one
week the temperature was slowly low-
ered until it reaehed 1,112° F. The
glass was held at this point for about
four days to allow the temperature of
the glass and furnaee to beeome uniform
throughout. At 1,112l° F. this partieu-
lar kind of glass (boro-silieate erown) is
quite rigid and yet suffieiently viseous to
yield to eooling stresses without danger
of eraeking. Beginning on May 18, the glass was
allowed to tool slowly at an average rate
Of 4I/2° F. per day till 860° F. was
temperature for six weeks during whieh
time no variation greater than 2° F. was
permitted. Final tooling was started
on August 30 and room temperature was
attained on January 16. The eover was
finally removed on January 21 and the
job was done ! During all these months there was no
assuranee as to what would be found in
the annealing furnaee when the final
" unveiling " took plaee, beeause the
slightest bit of foreign matter in the
glass might start radial eraeks whieh
would split the glass into fragments.
Too sudden ehanges in temperature
would be equally serious. Mr. Finn and
his assistants ean now take pride in the
faet that the bureau has aeeomplished a
unique and sueeessful pieee of work.
Dr. George E. Burgess, direetor of the
bureau, points out that the experienee
gained for the seientifie staff will be of
great advantage to the bureau and to
any American glass makers who may
wish to profit by the results, beeause all
the information obtained is ultimately
available to the Ameriean publie.
The next step toward eompletion of
this refleetor is to drill a hole at its op-
tieal axis. Although the majority of op-
tieians would refuse to grind a hole in a
pieee of glass so large as this, exeept at
the owner's risk, on aeeount of the pos-
sibility of breaking it, the experts at the
Bureau of Standards feel so eonfident
that this ean be done without any danger
that they will do it at their own risk.
After this the disk will be sent to an
optieian to be ground, polished and "fig-
ured" on one surfaee until it beeomes
parabolie. Sueh a surfaee has the prop-
erty of refleeting parallel rays of light to
a foeus. It is the eurve used in automo-
bile headlight refleetors, only in the ease
of a teleseope mirror the eurve is so
slight that it is hardly notieeable. With the eompletion of this disk, Pro-
fessor Perkins ' dream of an all-Amer-
iean teleseope will soon be realized.
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