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 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.