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 of this tremendous mirror, six feet across, and about four or five inches thick.  The dimensions were far in excess of those which had been contemplated in any previous attempt of the same kind.  Herschel had no doubt fashioned one mirror of four feet in diameter, and many others of smaller dimensions, but the processes which he employed had never been fully published, and it was obvious that, with a large increase in dimensions, great additional difficulties had to be encountered.  Difficulties began at the very commencement of the process, and were experienced in one form or another at every subsequent stage.  In the first place, the mere casting of a great disc of this mixture of tin and copper, weighing something like three or four tons, involved very troublesome problems.  No doubt a casting of this size, if the material had been, for example, iron, would have offered no difficulties beyond those with which every practical founder is well acquainted, and which he has to encounter daily in the course of his ordinary work.  But speculum metal is a material of a very intractable description.  There is, of course, no practical difficulty in melting the copper, nor in adding the proper proportion of tin when the copper has been melted.  There may be no great difficulty in arranging an organization by which several crucibles, filled with the molten material, shall be poured simultaneously so as to obtain the requisite mass of metal, but from