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.