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Equilateral Glass Prism: Sawn, Ground and Pitch-Polished
Frankford Arsenal, in F. E. Wright, The Manufacture of Optical Glass and of Optical Systems (1921), Fig. 77 · Public domain (US Government publication)
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Charlie

19. Oṣù Kẹwà 2026DE
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Equilateral Glass Prism: Sawn, Ground and Pitch-Polished

A prism splits white light into its colours, so its faces must be optically flat and its angles true. This is how the US arsenals ground prisms in 1918, as F. E. Wright recorded for the Ordnance Department in 1921: saw a blank from optical glass, rough-grind it on iron disks, set a batch of prisms in plaster, fine-grind, and polish on pitch with rouge. Wright's worked example is a right-angle prism; an equilateral prism has three 60° angles, made the same way. The picture: blocking prisms for rough grinding at Frankford Arsenal (Wright 1921, Fig. 77, public domain).
Ilọsíwájú
2-3 days

Ìlànà

1

Saw out the blank

Saw a triangular blank from a block of optical glass with a diamond saw, using kerosene as lubricant. Leave several millimetres extra on each face for grinding.

Àwọn ohun èlò fún ìgbésẹ̀ yìí:

Gilasi OpitikaGilasi Opitika1 as required

Àwọn irinṣẹ́ tí a nílò:

Ayùn Dáyámọ́ndìAyùn Dáyámọ́ndì
Kẹ́rọ́sínìKẹ́rọ́sínì
Òṣùwọ̀n Kálípà Díjítà Ìnṣì 6Òṣùwọ̀n Kálípà Díjítà Ìnṣì 6
Gíláàsì ààbòGíláàsì ààbò
2

Rough-grind the faces

Grind one triangular end flat on a rotating iron disk with 90 or 150 mesh grit. Then rough out the three side faces at right angles to it, checking the 60° angles with an angle gauge.

Àwọn irinṣẹ́ tí a nílò:

Àwo Lílọ IrinÀwo Lílọ Irin
Ìṣù èérún ìlọ̀ silicon carbideÌṣù èérún ìlọ̀ silicon carbide
Wọ̀n IgunWọ̀n Igun
3

Grind the ends in a batch

Wax 20 or more prisms by their ground ends onto a heated iron disk and grind the other ends together: 90, 150, then 220 mesh, then smoothing grit. Change grit only when the surface is free of scratches. Regrind to the set thickness.

Àwọn irinṣẹ́ tí a nílò:

Àwo Lílọ IrinÀwo Lílọ Irin
Ìṣù èérún ìlọ̀ silicon carbideÌṣù èérún ìlọ̀ silicon carbide
Ìdí parafíìnìÌdí parafíìnì
Ìdí OyinÌdí Oyin
Pátákó GbígbónáPátákó Gbígbóná
Òṣùwọ̀n Kálípà Díjítà Ìnṣì 6Òṣùwọ̀n Kálípà Díjítà Ìnṣì 6
4

Block the prisms in plaster

Set the prisms face down in melted paraffin on a flat iron disk and coat their backs with beeswax. Clamp a ring round them, fill with plaster of Paris and a little Portland cement (10 : 1), and let it harden about 16 hours. Cut the plaster back 1/8 in from the prisms and give it two coats of shellac.

Àwọn irinṣẹ́ tí a nílò:

Pátákó GbígbónáPátákó Gbígbóná
Ìdí parafíìnìÌdí parafíìnì
Ìdí OyinÌdí Oyin
Pilasita ti ParisPilasita ti Paris
Símẹ́ǹtì PortlandSímẹ́ǹtì Portland
Èròjà Ìdí OyinÈròjà Ìdí Oyin
5

Fine-grind

Grind the block on a slightly convex plate with wet flour emery (the part that settles out of water between 10 and 60 minutes). After five minutes check with a magnifier: the faces should have an even velvet finish, slightly hollow by less than 0.001 in.

Àwọn irinṣẹ́ tí a nílò:

Àwo Lílọ IrinÀwo Lílọ Irin
Ìṣù èérún ìlọ̀ silicon carbideÌṣù èérún ìlọ̀ silicon carbide
Lúpù ÌgbégaLúpù Ìgbéga
6

Polish on pitch

Pour strained pitch with a little rosin 1/4 in deep on an iron tool and cut crossed grooves 1/8 in wide, 1 in apart. Press it true with a rouge-wetted flat. Polish the block with creamy rouge and water on the machine, usually 4–8 hours, running nearly dry at the end.

Àwọn irinṣẹ́ tí a nílò:

Ọ̀dà igi tí ó leỌ̀dà igi tí ó le
Ọ̀dà igi painiỌ̀dà igi paini
Pupa ìdán alágbẹ̀dẹ wúràPupa ìdán alágbẹ̀dẹ wúrà
Ẹ̀rọ Dídán OpitikaẸ̀rọ Dídán Opitika
7

Test for flatness

Lay an optical flat on each face and look at the Newton colours in sodium light (salt in a Bunsen flame). Straight bands to the edge, or one colour over the whole face, mean it is flat.

Àwọn irinṣẹ́ tí a nílò:

Ojú pẹ̀lẹ́bẹ́ optiki (ìtọ́kasí)Ojú pẹ̀lẹ́bẹ́ optiki (ìtọ́kasí)
Ọ̀gọ̀ BunsenỌ̀gọ̀ Bunsen
Iyọ̀Iyọ̀
Lúpù ÌgbégaLúpù Ìgbéga
8

Free, clean, check the angle, repeat

Break the plaster away, clean the prisms with benzene and check each angle with the angle gauge. Correct by hand on a flat tool with smoothing grit. Repeat blocking and polishing for every face, smallest face first.

Àwọn irinṣẹ́ tí a nílò:

ÒòlùÒòlù
BenzeneBenzene
Wọ̀n IgunWọ̀n Igun
Àwo Lílọ IrinÀwo Lílọ Irin
Ìṣù èérún ìlọ̀ silicon carbideÌṣù èérún ìlọ̀ silicon carbide

Àwọn ohun-èlò

1

Àwọn irinṣẹ́ tó nílò

23

Ìtọ́nisọ́nà yìí ń ṣe

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Àyè

Prism gíláàsì (ẹgbẹ́ dọ́gba)

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Blueprint tó jọra

Àwọn blueprint wọ̀nyí pín ìmọ̀ — ọ̀nà, ohun-èlò tàbí ìlànà

Optical Glass: Borosilicate Crown Melted, Stirred and Cooled in the Pot
Léwu

Optical Glass: Borosilicate Crown Melted, Stirred and Cooled in the Pot

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