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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).
高级
2-3 days
说明
1
1
Saw out the blank
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.
此步骤所需材料：
光学玻璃1 as required所需工具：
金刚石锯
煤油
6 英寸数显卡尺
安全眼镜2
2
Rough-grind the faces
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.
所需工具：
铸铁研磨盘
碳化硅磨料粒度套装
角度规3
3
Grind the ends in a batch
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.
所需工具：
铸铁研磨盘碳化硅磨料粒度套装
石蜡
蜂蜡
电热板6 英寸数显卡尺4
4
Block the prisms in plaster
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.
所需工具：
电热板石蜡蜂蜡
熟石膏
硅酸盐水泥
蜂蜡颗粒5
5
Fine-grind
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.
所需工具：
铸铁研磨盘碳化硅磨料粒度套装
放大目镜6
6
Polish on pitch
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.
所需工具：
木焦油沥青
松香
首饰红抛光膏
光学抛光机7
7
Test for flatness
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.
所需工具：
光学平晶（基准）
本生灯
盐放大目镜8
8
Free, clean, check the angle, repeat
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.
所需工具：
锤子
苯角度规铸铁研磨盘碳化硅磨料粒度套装此蓝图制作
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