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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. uMfumfu 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).
Osezingeni eliphezulu
2-3 days

Imiyalelo

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.

Izinto zokwakha zalesi sinyathelo:

Ingilazi YeoptikiIngilazi Yeoptiki1 as required

Amathuluzi adingekayo:

Isaha YedayimaneIsaha Yedayimane
IpharafiniIpharafini
I-Caliper Yedijithali Yamayintshi Ayi-6I-Caliper Yedijithali Yamayintshi Ayi-6
Izibuko zokuphephaIzibuko zokuphepha
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.

Amathuluzi adingekayo:

Idiski Yensimbi YokugayaIdiski Yensimbi Yokugaya
Isethi yezinhlayiya zokugaya ze-silicon carbideIsethi yezinhlayiya zokugaya ze-silicon carbide
Isilinganisi SekhoniIsilinganisi Sekhoni
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.

Amathuluzi adingekayo:

Idiski Yensimbi YokugayaIdiski Yensimbi Yokugaya
Isethi yezinhlayiya zokugaya ze-silicon carbideIsethi yezinhlayiya zokugaya ze-silicon carbide
I-paraffin waxI-paraffin wax
Isibabule SezinyosiIsibabule Sezinyosi
Ipuleti ElishisayoIpuleti Elishisayo
I-Caliper Yedijithali Yamayintshi Ayi-6I-Caliper Yedijithali Yamayintshi Ayi-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.

Amathuluzi adingekayo:

Ipuleti ElishisayoIpuleti Elishisayo
I-paraffin waxI-paraffin wax
Isibabule SezinyosiIsibabule Sezinyosi
Ipulasta YaseParisIpulasta YaseParis
Usimende we-PortlandUsimende we-Portland
Amaphilisi Omovu WezinyosiAmaphilisi Omovu Wezinyosi
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.

Amathuluzi adingekayo:

Idiski Yensimbi YokugayaIdiski Yensimbi Yokugaya
Isethi yezinhlayiya zokugaya ze-silicon carbideIsethi yezinhlayiya zokugaya ze-silicon carbide
I-Loupe YokukhulisaI-Loupe Yokukhulisa
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.

Amathuluzi adingekayo:

Itiyela lomuthi elijiyileItiyela lomuthi elijiyile
I-rosin yephayiniI-rosin yephayini
Ubomvu bomenzi wobucwebeUbomvu bomenzi wobucwebe
Umshini Wokupholisha IyoptikiUmshini Wokupholisha Iyoptiki
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.

Amathuluzi adingekayo:

Ubuso obupholile be-optical (okokubhekisela)Ubuso obupholile be-optical (okokubhekisela)
Isibani se-BunsenIsibani se-Bunsen
UsawotiUsawoti
I-Loupe YokukhulisaI-Loupe Yokukhulisa
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.

Amathuluzi adingekayo:

IsandoIsando
I-benzeneI-benzene
Isilinganisi SekhoniIsilinganisi Sekhoni
Idiski Yensimbi YokugayaIdiski Yensimbi Yokugaya
Isethi yezinhlayiya zokugaya ze-silicon carbideIsethi yezinhlayiya zokugaya ze-silicon carbide

Izinto

1

Amathuluzi Adingekayo

23

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I-prism yengilazi (enezinhlangothi ezilinganayo)

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

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