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UBWIZA N'UBUZIMA
UBUKORIKORI
UMUCO N'AMATEKA
IMYIDAGADURO
IBIDUKIKIJE
IBIRIBWA N'IBINYOBWA
UBWUBATSI BUSUBIRA INYUMA
UBUMENYI
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IBYAMBARWA
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)
Charlie

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Charlie

19. Ukwakira 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).
Hejuru
2-3 days

Amabwiriza

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.

Ibikoresho by'iyi ntambwe:

Ikirahure cya OptikeIkirahure cya Optike1 as required

Ibikoresho bikenewe:

Umusumeno wa DiyamaUmusumeno wa Diyama
PeteroliPeteroli
Ikigereranyo cya Digitale cy'Amasentimetero 6Ikigereranyo cya Digitale cy'Amasentimetero 6
Amadarubindi y'umutekanoAmadarubindi y'umutekano
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.

Ibikoresho bikenewe:

Disiki y’Icyuma yo GusyaDisiki y’Icyuma yo Gusya
Ikusanyirizo ry'uducye tw'imisya twa karubide ya silisiyumuIkusanyirizo ry'uducye tw'imisya twa karubide ya silisiyumu
Igipimo cy’InguniIgipimo cy’Inguni
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.

Ibikoresho bikenewe:

Disiki y’Icyuma yo GusyaDisiki y’Icyuma yo Gusya
Ikusanyirizo ry'uducye tw'imisya twa karubide ya silisiyumuIkusanyirizo ry'uducye tw'imisya twa karubide ya silisiyumu
Ibuki rya parafineIbuki rya parafine
IgishasharaIgishashara
Urubaho rushyushyaUrubaho rushyushya
Ikigereranyo cya Digitale cy'Amasentimetero 6Ikigereranyo cya Digitale cy'Amasentimetero 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.

Ibikoresho bikenewe:

Urubaho rushyushyaUrubaho rushyushya
Ibuki rya parafineIbuki rya parafine
IgishasharaIgishashara
Ipuratsi ya ParisIpuratsi ya Paris
Sima ya porutilandeSima ya porutilande
Uduce tw'IgishasharaUduce tw'Igishashara
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.

Ibikoresho bikenewe:

Disiki y’Icyuma yo GusyaDisiki y’Icyuma yo Gusya
Ikusanyirizo ry'uducye tw'imisya twa karubide ya silisiyumuIkusanyirizo ry'uducye tw'imisya twa karubide ya silisiyumu
Lupu YaguraLupu Yagura
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.

Ibikoresho bikenewe:

Gudurona y'ibiti ikomeyeGudurona y'ibiti ikomeye
Ubukomezi bw'igiti cya pinusiUbukomezi bw'igiti cya pinusi
Umutuku w'umucuziUmutuku w'umucuzi
Imashini yo Kunoza OptikeImashini yo Kunoza Optike
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.

Ibikoresho bikenewe:

Ubuso buringaniye bw'ibirahure (ngenderwaho)Ubuso buringaniye bw'ibirahure (ngenderwaho)
Itara rya BunsenItara rya Bunsen
UmunyuUmunyu
Lupu YaguraLupu Yagura
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.

Ibikoresho bikenewe:

InyundoInyundo
BenzeneBenzene
Igipimo cy’InguniIgipimo cy’Inguni
Disiki y’Icyuma yo GusyaDisiki y’Icyuma yo Gusya
Ikusanyirizo ry'uducye tw'imisya twa karubide ya silisiyumuIkusanyirizo ry'uducye tw'imisya twa karubide ya silisiyumu

Ibikoresho

1

Ibikoresho bikenewe

23

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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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