Great Astronomers by Robert Stawell Ball (best ereader for students .txt) 📖

For many years, however, he continued to labour at his

original calling, nor was it until he had attained middle age and

become the most celebrated astronomer of the time, that he was

enabled to concentrate his attention exclusively on his favourite

pursuit.

 

It was with quite a small telescope which had been lent him by a

friend that Herschel commenced his career as an observer.

However, he speedily discovered that to see all he wanted to see,

a telescope of far greater power would be necessary, and he

determined to obtain this more powerful instrument by actually

making it with his own hands. At first it may seem scarcely

likely that one whose occupation had previously been the study and

practice of music should meet with success in so technical an

operation as the construction of a telescope. It may, however, be

mentioned that the kind of instrument which Herschel designed to

construct was formed on a very different principle from the

refracting telescopes with which we are ordinarily familiar. His

telescope was to be what is termed a reflector. In this type of

instrument the optical power is obtained by the use of a mirror at

the bottom of the tube, and the astronomer looks down through the

tube TOWARDS HIS MIRROR and views the reflection of the stars

with its aid. Its efficiency as a telescope depends entirely on

the accuracy with which the requisite form has been imparted to

the mirror. The surface has to be hollowed out a little, and this

has to be done so truly that the slightest deviation from good

workmanship in this essential particular would be fatal to

efficient performance of the telescope.

 

[PLATE: WILLIAM HERSCHEL.]

 

The mirror that Herschel employed was composed of a mixture of two

parts of copper to one of tin; the alloy thus obtained is an

intensely hard material, very difficult to cast into the proper

shape, and very difficult to work afterwards. It possesses,

however, when polished, a lustre hardly inferior to that of silver

itself. Herschel has recorded hardly any particulars as to the

actual process by which he cast and figured his reflectors.

We are however, told that in later years, after his telescopes had

become famous, he made a considerable sum of money by the

manufacture and sale of great instruments. Perhaps this may be

the reason why he never found it expedient to publish any very

explicit details as to the means by which his remarkable

successes were obtained.

 

[PLATE: CAROLINE HERSCHEL.]

 

Since Herschel’s time many other astronomers, notably the late

Earl of Rosse, have experimented in the same direction, and

succeeded in making telescopes certainly far greater, and probably

more perfect, than any which Herschel appears to have constructed.

The details of these later methods are now well known, and have

been extensively practised. Many amateurs have thus been able to

make telescopes by following the instructions so clearly laid

down by Lord Rosse and the other authorities. Indeed, it would

seem that any one who has a little mechanical skill and a good

deal of patience ought now to experience no great difficulty in

constructing a telescope quite as powerful as that which first

brought Herschel into fame. I should, however, mention that in

these modern days the material generally used for the mirror is

of a more tractable description than the metallic substance which

was employed by Herschel and by Lord Rosse. A reflecting

telescope of the present day would not be fitted with a mirror

composed of that alloy known as speculum metal, whose composition

I have already mentioned. It has been found more advantageous to

employ a glass mirror carefully figured and polished, just as a

metallic mirror would have been, and then to impart to the

polished glass surface a fine coating of silver laid down by a

chemical process. The silver-on-glass mirrors are so much lighter

and so much easier to construct that the more old-fashioned

metallic mirrors may be said to have fallen into almost total

disuse. In one respect however, the metallic mirror may still

claim the advantage that, with reasonable care, its surface will

last bright and untarnished for a much longer period than can the

silver film on the glass. However, the operation of re-silvering

a glass has now become such a simple one that the advantage this

indicates is not relatively so great as might at first be supposed.

 

[PLATE: STREET VIEW, HERSCHEL HOUSE, SLOUGH.]

 

Some years elapsed after Herschel’s attention had been first

directed to astronomy, before he reaped the reward of his

exertions in the possession of a telescope which would adequately

reveal some of the glories of the heavens. It was in 1774, when

the astronomer was thirty-six years old, that he obtained his

first glimpse of the stars with an instrument of his own

construction. Night after night, as soon as his musical labours

were ended, his telescopes were brought out, sometimes into the

small back garden of his house at Bath, and sometimes into the

street in front of his hall-door. It was characteristic of him

that he was always endeavouring to improve his apparatus. He was

incessantly making fresh mirrors, or trying new lenses, or

combinations of lenses to act as eye-pieces, or projecting

alterations in the mounting by which the telescope was supported.

Such was his enthusiasm that his house, we are told, was

incessantly littered with the usual indications of the workman’s

presence, greatly to the distress of his sister, who, at this

time, had come to take up her abode with him and look after his

housekeeping. Indeed, she complained that in his astronomical

ardour he sometimes omitted to take off, before going into his

workshop, the beautiful lace ruffles which he wore while

conducting a concert, and that consequently they became soiled

with the pitch employed in the polishing of his mirrors.

 

This sister, who occupies such a distinct place in scientific

history is the same little girl to whom we have already referred.

From her earliest days she seems to have cherished a passionate

admiration for her brilliant brother William. It was the

proudest delight of her childhood as well as of her mature years

to render him whatever service she could; no man of science was

ever provided with a more capable or energetic helper than

William Herschel found in this remarkable woman. Whatever work

had to be done she was willing to bear her share in it, or even to

toil at it unassisted if she could be allowed to do so. She not

only managed all his domestic affairs, but in the grinding of the

lenses and in the polishing of the mirrors she rendered every

assistance that was possible. At one stage of the very delicate

operation of fashioning a reflector, it is necessary for the

workman to remain with his hand on the mirror for many hours in

succession. When such labours were in progress, Caroline used to

sit by her brother, and enliven the time by reading stories aloud,

sometimes pausing to feed him with a spoon while his hands were

engaged on the task from which he could not desist for a moment.

 

When mathematical work had to be done Caroline was ready for it;

she had taught herself sufficient to enable her to perform the

kind of calculations, not, perhaps, very difficult ones, that

Herschel’s work required; indeed, it is not too much to say that

the mighty life-work which this man was enabled to perform could

never have been accomplished had it not been for the self-sacrifice of this ever-loving and faithful sister. When Herschel

was at the telescope at night, Caroline sat by him at her desk,

pen in hand, ready to write down the notes of the observations as

they fell from her brother’s lips. This was no insignificant

toil. The telescope was, of course, in the open air, and as

Herschel not unfrequently continued his observations throughout

the whole of a long winter’s night, there were but few women who

could have accomplished the task which Caroline so cheerfully

executed. From dusk till dawn, when the sky was clear, were

Herschel’s observing hours, and what this sometimes implied we can

realise from the fact that Caroline assures us she had sometimes

to desist because the ink had actually frozen in her pen. The

night’s work over, a brief rest was taken, and while William had

his labours for the day to attend to, Caroline carefully

transcribed the observations made during the night before,

reduced all the figures and prepared everything in readiness for

the observations that were to follow on the ensuing evening.

 

But we have here been anticipating a little of the future which

lay before the great astronomer; we must now revert to the

history of his early work, at Bath, in 1774, when Herschel’s

scrutiny of the skies first commenced with an instrument of his

own manufacture. For some few years he did not attain any result

of importance; no doubt he made a few interesting observations,

but the value of the work during those years is to be found, not

in any actual discoveries which were accomplished, but in the

practice which Herschel obtained in the use of his instruments.

It was not until 1782 that the great achievement took place by

which he at once sprang into fame.

 

[PLATE: GARDEN VIEW, HERSCHEL HOUSE, SLOUGH.]

 

It is sometimes said that discoveries are made by accident,

and, no doubt, to a certain extent, but only, I fancy to a

very small extent, this statement may be true. It is, at all

events, certain that such lucky accidents do not often fall to

the lot of people unless those people have done much to deserve

them. This was certainly the case with Herschel. He appears to

have formed a project for making a close examination of all the

stars above a certain magnitude. Perhaps he intended to confine

this research to a limited region of the sky, but, at all events,

he seems to have undertaken the work energetically and

systematically. Star after star was brought to the centre of the

field of view of his telescope, and after being carefully examined

was then displaced, while another star was brought forward to be

submitted to the same process. In the great majority of cases

such observations yield really nothing of importance; no doubt

even the smallest star in the heavens would, if we could find out

all about it, reveal far more than all the astronomers that were

ever on the earth have even conjectured. What we actually learn

about the great majority of stars is only information of the most

meagre description. We see that the star is a little point of

light, and we see nothing more.

 

In the great review which Herschel undertook he doubtless examined

hundreds, or perhaps thousands of stars, allowing them to pass

away without note or comment. But on an ever-memorable night in

March, 1782, it happened that he was pursuing his task among

the stars in the Constellation of Gemini. Doubtless, on that

night, as on so many other nights, one star after another was

looked at only to be dismissed, as not requiring further

attention. On the evening in question, however, one star was

noticed which, to Herschel’s acute vision seemed different from

the stars which in so many thousands are strewn over the sky. A

star properly so called appears merely as a little point of light,

which no increase of magnifying power will ever exhibit with a

true disc. But there was something in the star-like object which

Herschel saw that immediately arrested his attention and made him

apply to it a higher magnifying power. This at once disclosed the

fact that the object possessed a disc, that is, a definite,

measurable size, and that it was thus totally different