skies. But in Lord Rosse's telescope you would look in vain for
these glasses, and it is not at the lower end of the instrument that
you are to take your station when you are going to make your
observations. The astronomer at Parsonstown has rather to avail
himself of the ingenious system of staircases and galleries, by which
he is enabled to obtain access to the mouth of the great tube. The
colossal telescope which swings between the great walls, like
Herschel's great telescope already mentioned, is a reflector, the
original invention of which is due of course to Newton. The optical
work which is accomplished by the lenses in the ordinary telescope is
effected in the type of instrument constructed by Lord Rosse by a
reflecting mirror which is placed at the lower end of the vast tube.
The mirror in this instrument is made of a metal consisting of two
parts of copper to one of tin. As we have already seen, this mixture
forms an alloy of a very peculiar nature. The copper and the tin
both surrender their distinctive qualities, and unite to form a
material of a very different physical character. The copper is tough
and brown, the tin is no doubt silvery in hue, but soft and almost
fibrous in texture. When the two metals are mixed together in the
proportions I have stated, the alloy obtained is intensely hard and
quite brittle being in both these respects utterly unlike either of
the two ingredients of which it is composed. It does, however,
resemble the tin in its whiteness, but it acquires a lustre far
brighter than tin; in fact, this alloy hardly falls short of silver
itself in its brilliance when polished.

[PLATE: LORD ROSSE'S TELESCOPE. From a photograph by W. Lawrence,
Upper Sackville Street, Dublin.]

The first duty that Lord Rosse had to undertake was the construction