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They're fairly common now.

But back in the '60s, when they were introduced, photochromic

sunglasses - they turned dark in bright sunlight - made a bit of

a splash. Materials that behaved like that were quite uncommon.

But some bloke did the research and came up with the following,

rather amazing, piece of information. Photochromic materials

are reportedly several millenia old, dating to the time of Alexander

the Great. This is a summary of a piece that appeared in 1966

in the IEEE Spectrum technical magazine. What you're about to

read may astound you.

Most people think of photochromic materials as a recent invention.

But records indicate that this is not so. It is recorded that the soldiers

of Alexander the Great knew of a black substance which could be

ground into a powder, dissolved in water and then soaked into

swatches of cloth torn from their uniforms.

When tied around a soldier's wrist, and as the sun rose, then sat

high in the sky, and later set in the West, the cloth would change

colors in a subtle way. With practice, the unaided eye could discern

these color changes and from them deduce quickly the approximate

time of day. That invention became widely known as Alexander's Rag

Time Band.

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2 answers to this question

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Should be in jokes me thinks, and now you have me thinking!

There should be a section for these.

Lets hope your post produces more.

Instant five star rating!

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