e each of these lenses is itself made up
of two, we should more correctly say four lenses in all. It was with a
lens of this kind that the comet pictures had been taken at the Cape of
Good Hope, and it might seem the safest plan to adopt what had been shown
to be capable of such good work. But there was this difficulty; the
pictures of the comet were on a very small scale, and taken with a small
lens; a much larger lens was required for the scheme now under
contemplation, and when there are four separate lenses to be made, each
with two surfaces to polish, and each requiring a perfectly sound clear
piece of glass, it will be obvious that the difficulties of making a large
compound lens of this kind are much greater, and the expense much more
serious than in the case of a single lens, or even a pair. It was this
question of expense which had led the brothers Henry to experiment with a
different kind of instrument, in which only one pair of lenses was used
instead of two. Their instrument was, in fact, very similar to the
ordinary telescope, excepting that they were bound to make their lenses
somewhat different in shape in order to bring to focus the rays of light
suitable for photography, which are not the same as those suitable for eye
observation with the ordinary telescope. Dr. Common, again, had used a
third kind of instrument, mainly with the view of reducing the necessary
expense still further, or, perhaps, of increasing the size of the
instrument for the same expense. His telescope had no lens at all, but a
curved mirror instead, the mirror being made of glass silvered on the face
(not on the back as in the ordinary looking-glass). In this case there is
only one surface to polish instead of four, as in the Henrys' telescope,
or eight, as in the case of the photographic doublet; and, moreover, since
the rays of light are reflected from the surface of the glass, and do not
pass _through_ it at all, the internal structure of the glass is not so
strictly important as in the other cases. Hence the reflector is a very
cheap instrument, and it is, moreover, quite free from some difficulties
attached to the other instruments. No correction for rays of light of
different colours is required, since all rays of whatever colour come to
the same focus automatically. These advantages of the reflector were so
considerable as to almost outweigh one well-known disadvantage, which is,
however, not very easily expressed in words. The r