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  • I am holding probably the most amazing piece of metal that you have seen in any of our videos.

  • This is the original mirror from Isaac Newton's first telescope

  • and this is his actual first telescope that he made.

  • You can see behind it is the drawings that he made

  • and the description he wrote in Latin of this telescope

  • So

  • this telescope

  • has been with the Royal Society

  • since 1688.

  • There was a bit of interruption, where it was,

  • where it went to an instrument maker

  • so there may be some later additions,

  • but there is absolutely no doubt

  • that this mirror is the original mirror.

  • If I was a physicist

  • I'd be telling you about the

  • wonders of this telescope

  • and the advantage of using a

  • mirror over lenses

  • so you get the images.

  • But I'm going to tell you

  • about the metal of the mirror

  • and the mirror is made from a metal

  • that is called speculum.

  • Speculum is not an element,

  • it's an alloy;

  • a mixture of metals

  • and

  • it's a mixture

  • of tin and copper.

  • Now you can see the problem straight away

  • if you're making a mirror

  • to look at stars and things

  • is that copper is coloured;

  • it's red,

  • or reddish colour.

  • And for a really good mirror

  • you need to have it absolutely colourless

  • so you see the actual colours of whatever is in space.

  • The moon looks the right colour.

  • It just happens that if you mix

  • two parts of copper with one part of tin

  • and just put in a little amount of arsenic,

  • and obviously I am not in the lab so I haven't got any arsenic here,

  • you can get a metal which has been known for

  • perhaps a couple of thousand years called speculum,

  • which gives you a really nice mirror finish.

  • Speculum is the Latin word for a mirror

  • [BRADY]: That's not a very good mirror

  • [PROF POLIAKOFF]: Well, when it was first made it would have been polished

  • and it would have been really good and

  • you can still see the traces of its polishing.

  • So what's amazing about this

  • is how heavy the metal is.

  • Copper is quite a dense metal

  • and so is tin so this is much heavier

  • than the modern mirrors that you would have, which are made out of aluminium, which is very light.

  • And this mirror fits in the back here

  • so the light comes into there

  • onto the mirror and back again

  • and here is the eye piece so you look in there.

  • And if you look at Isaac Newton's drawing here

  • you can see there is a picture of his eye.

  • I don't know whether this is Isaac Newton's eye or somebody else's.

  • So you can see here is a large ball,

  • which is quite a clever mount,

  • that allows you to turn the telescope in whatever direction you want.

  • And I was quite mystified because

  • there is a little crown down here

  • and

  • I wondered why on Earth in a telescope you should need a crown.

  • And our librarian explained to me

  • that Newton tested his telescope

  • by looking at some architectural feature in Cambridge

  • on another building,

  • and

  • here is an indication

  • that he drew to show the magnification;

  • how much bigger the image was,

  • and this is what it looked like to the naked eye,

  • and here is what it looked like through his telescope.

  • So it really worked

  • I just think it's extraordinary

  • that I should be touching something

  • that Isaac Newton should have made himself.

  • But

  • to come back to the chemistry

  • it is really very interesting

  • that one of the key features of this telescope

  • was having the right material to make the mirror because

  • if you couldn't make the mirror, nothing else is important.

  • And so

  • right at the heart of this fundamental development

  • in physics

  • was a bit of chemistry.

  • It turns out that you have to be quite clever in making this alloy,

  • because if you put too much copper then it's red,

  • and surprisingly if you put too much tin it's a bit blue.

  • So there is a rather nice sweet spot

  • which, depending how much arsenic you put in,

  • gives you just the right mixture

  • so that the surface reflects colours

  • faithfully and doesn't absorb anywhere in the visible spectrum.

  • I'm not a metallurgist, but very often

  • quite small amounts of added material

  • can affect the way that metals crystallise

  • and the way that it crystallises then affects its surface properties

  • and hence affects the way that it reflects light.

I am holding probably the most amazing piece of metal that you have seen in any of our videos.

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B1 中級

驚きの金属片(鏡 (Amazing piece of metal (speculum))

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    林宜悉 に公開 2021 年 01 月 14 日
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