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music, and thus he was led to study mathematics. When he had once tasted the charms of
mathematics, he saw vast regions of knowledge unfolded before him, and in this way he was
induced to direct his attention to astronomy. More and more this pursuit seems to have
engrossed his attention, until at last it had become an absorbing passion. Herschel was,
however, still obliged, by the exigency of procuring a livelihood, to give up the best part of his
time to his profession as a musician; but his heart was eagerly fixed on another science, and
every spare moment was steadily devoted to astronomy. 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
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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.
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.
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.
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
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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 [ Pobierz całość w formacie PDF ]