I have to begin the account of this discovery by telling you a little
story. In the middle of the last century there lived at Hanover a
teacher of music whose name was Isaac Herschel. He had a family of ten
children, and he did the best for them that his scanty means would
permit. Of his children William was the fourth, and he inherited his
father’s talents for music, as did most of his brothers and sisters.
He was a bright, clever boy at school, and he made such good progress
in his music that by the time he was fourteen years old he was able
to play in the military band of the Hanoverian Guards. War broke out
between France and England, and as Hanover was then under the English
crown, the French invaded it, and a battle was fought in which the poor
Hanoverian Guards suffered very terribly. Young Herschel spent the
night after the battle in a ditch, and he came to the conclusion that
he did not like fighting, though he was only a member of the band, and
he resolved to change his profession. That was not so easy to do just
then, for even a bandsman cannot leave the service in war time at his
own free will. William Herschel, however, showed all through his life
that he was not the man to be baffled by difficulties. I do not know
whether he asked for leave, but at all events he took it. He deserted,
in fact, and his friends succeeded in sending him away to England.
He was nineteen years old when he commenced to look for a career over
here, and certainly he found his prospects in the musical profession
very discouraging. Herschel was, however, very industrious; and at last
he succeeded in getting appointed as organist of the Octagon Chapel at
Bath. He gradually became famous for his musical skill, and had numbers
of pupils. He used also to conduct concerts and oratorios, and was
well known in this way over the West of England. Busy as Herschel was
with his profession, he still retained his love of reading and study.
Every moment he could spare from his duties he devoted to his books.
It was natural that a musician should specially desire to study the
theory of music, and to understand it properly you should know Euclid
and algebra, and, indeed, higher branches of mathematics as well.
Herschel did not know these things at first; he had not the means of
learning them when he was a boy, so he worked very hard after he became
a man. And he studied with such success that he made fair progress in
mathematics, and then it appeared to him that it would be interesting
to learn something about astronomy. After he had begun to read about
the stars, he thought he would like to see them, and so he borrowed a
telescope. It was only a little instrument, but it delighted him so
much that he said he must have one for himself. So he wrote to London
to make inquiries.
Telescopes were much dearer in those days than they are now, and
Herschel could not give the price that the opticians demanded. Here
again his invincible determination came to his aid. What was there to
prevent him from making a telescope? he asked himself; and forthwith
he began the attempt. You will think it strange, perhaps, that a
music-teacher who had no special training as a mechanic should at once
commence so delicate and difficult a task; but it is not really so hard
to make a telescope as might be imagined. The amateur cannot make such
a pretty-looking instrument as he is able to buy at the shops--the
tubes will not be so beautifully polished and the finish will be such
as a trained workman would be ashamed of--but the essential part of a
telescope is comparatively easy to make; at least, I should say of a
reflecting telescope, which is the kind Herschel attempted to make,
and succeeded in making. You must know that there are two kinds of
telescopes. The commoner one with which you are more familiar is called
the refracting telescope, and it has glass lenses. It was an instrument
constructed on this principle that we spoke of in a former lecture
(p. 97). The reflecting telescope depends for its power upon a bright
mirror at the lower end, and when using this instrument you look at
the reflection of the stars in the mirror. It was a reflector like
this that Herschel began to construct, and he engaged in the task with
enthusiasm. His sister Caroline had come to live with him, and she used
to help him at his work. So much in earnest was he that he used to rush
into his workshop directly he came home from a concert, and without
taking off his best clothes he would plunge into the grinding and
polishing of his mirrors. His sister tried to keep the house as tidy as
possible, but Herschel put up a carpenter’s shop in the drawing-room,
and turning-lathes in the best bedroom. At last he succeeded. He made
a mirror of the right shape, and found that it exhibited the stars
properly. It was not a looking-glass in the ordinary sense, with glass
on one side and quicksilver on the other. The mirror that Herschel
constructed was entirely of metal. It consisted of a mixture of two
parts of copper with one of tin.
[Illustration: FIG. 69.--The Mirror.]
The copper has first to be melted in a furnace, for the metal must be
above a red heat before it will begin to run. Then the tin has to be
carefully added, and the casting of the mirror is effected by pouring
the molten metal into a flat mould. Thus the rough mirror is obtained,
which in Herschel’s earlier telescopes seems to have been about six
or seven inches in diameter, and nearly an inch thick. Though copper
is such a tough substance, and though tin is also tough, yet when
melted together to make speculum metal, as this alloy is called, they
produce an exceedingly hard and brittle material. When we remember
that we could never break a copper penny piece by throwing it down on
the flags, it may seem strange that the “speculum metal” should be so
exceedingly brittle. A piece the size of a penny would be more brittle
than a bit of glass of the same dimensions, and when the speculum is
cast, unless it is cooled very carefully, it will fly into pieces.
Herein lay one of the difficulties that Herschel encountered. Speculum
metal must be put into an oven as soon as the casting has become solid,
and then the heat is gradually allowed to abate. When the speculum has
been at last obtained, next follows the labor of giving it the true
figure and polish. It is not only more fragile than glass, but it is
also quite as hard, and therefore the grinding is a tedious operation.
First the surface has to be ground with coarse sand, and then with
emery, which is gradually made finer and finer until the true figure
has been given (Fig. 69). The mirror is then somewhat basin-shaped,
but the depression is very slight. For example, in a mirror six inches
across the depression at the centre would perhaps be not more than the
twentieth of an inch. Small though this depression is, yet it has to be
made with exactness. In fact, if it were wrong at any point by so much
as the tenth of the thickness of this sheet of paper, the telescope
would not perform accurately. The tool that is used in grinding is
made of cast iron, and has been turned in a lathe to the right shape.
It is divided into squares in the manner shown in Fig. 70. After the
grinding comes the polishing, and this is effected with a tool like the
grinder in shape. This has to be covered over with little squares of
pitch, so that when warmed and put down on the mirror it is soft enough
to receive the right shape. Some rouge and water is spread over the
mirror, and the polisher is worked backward and forward with the hand
until a brilliant surface is obtained.
[Illustration: FIG. 70.--The Grinding Tool.]
When the amateur astronomer has completed this part of the task, all
the great difficulties about his telescope are conquered. The tube may
be made of wood, and, indeed, a square tube will do just as well as a
round one. He must also provide for the top of the tube a small mirror,
which has to be perfectly flat. The preparation of this requires much
care, because it is not so easy as one might suppose to obtain an
accurately flat surface. One way of doing this is to get three pieces,
and grind each two of them together until every pair will touch all
over; then they will certainly all be flat. One more part you want,
and that is an eyepiece. This presents no difficulty. A single glass
lens can be made to answer and your telescope is complete.