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Casting of an Optical Glass Disk

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Casting of an Optical Glass Disk Source: The Scientific Monthly, Vol. 26, No. 5 (May, 1928), pp. 479-480 Published by: American Association for the Advancement of Science Stable URL: http://www.jstor.org/stable/8017 . Accessed: 08/05/2014 02:43 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. . American Association for the Advancement of Science is collaborating with JSTOR to digitize, preserve and extend access to The Scientific Monthly. http://www.jstor.org This content downloaded from 169.229.32.137 on Thu, 8 May 2014 02:43:26 AM All use subject to JSTOR Terms and Conditions
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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 .

Accessed: 08/05/2014 02:43

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at .http://www.jstor.org/page/info/about/policies/terms.jsp

.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact [email protected].

.

American Association for the Advancement of Science is collaborating with JSTOR to digitize, preserve andextend access to The Scientific Monthly.

http://www.jstor.org

This content downloaded from 169.229.32.137 on Thu, 8 May 2014 02:43:26 AMAll use subject to JSTOR Terms and Conditions

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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