Seventy-five years since, the only planets known to men of science were the same which were known to the Chaldean shepherds thousands of years ago. Between the orbit of Mars and that of Jupiter, there occurs an interval of no less than 350 millions of miles, in which no planet was known to exist before the commencement of the present century. Nearly three centuries ago, Kepler had pointed out something like a regular progression in the distances of the planets as far as Mars, which was broken in the case of Jupiter: Having despaired of reconciling the actual state of the planetary system with any theory he could form respecting it, he hazarded the conjecture that a planet really existed between the orbits of Mars and Jupiter, and that its smallness alone prevented it from being visible to astronomers. The remarkable passage containing this conjecture is found in his Prodromus, and is as follows: “When this plan, therefore, failed, I tried to reach my aim in another way, of, I must confess, singular boldness. Between Jupiter and Mars I interposed a new planet, and another also between Venus and Mercury, both which it is possible are not visible on account of their minuteness, and I assigned to them their respective periods. In this way I thought that I might in some degree equalize their ratios, which ratios regularly diminished toward the sun, and enlarged toward the fixed stars.”

But Kepler himself soon rejected this idea as improbable, and it does not appear to have received any favor from the astronomers of that time.

To satisfy a curiosity that is naturally excited by any thing new, great or uncommon, I tried to obtain all the information I could, connected with the Telescopes I endeavour to describe. To gratify a similar feeling in others, more remote from my opportunities of looking on, I venture to publish an account of what I have seen.

As I am hardy enough to do so without any assistance from, or even the cognizance of the noble projector of those instruments, whose liberality in diffusing his knowledge and wish for its promotion, leave me no uneasiness on this point, so I do not expect to give that information which men of deep research or mathematically close enquiry would desire. There are some particulars which might, perhaps, be more enlarged upon with advantage, but it has been my aim to place before the general reader such an account as will make the manufacture of the Specula, and the mechanism of the Telescopes, as plainly understood as could be expected, without entering with too much tediousness into minute details. I have been as explicit as possible in the history of the compound three foot Speculum, knowing that individuals whose inclination would lead them to construct Specula on a large scale, without possessing the pecuniary advantages of Lord Rosse, will be naturally led to adopt a course the most manageable and economical, and one which does not appear to be the less certain of success.

The rapid sale of the First Edition, together with the many very favorable opinions expressed in reference to it by the Newspaper Press, leads the Author to hope that this work has been presented in an acceptable form. In the present edition he has therefore merely supplied a few trifling omissions which an unavoidable hurry in bringing out the former, occasioned.

In the account of the three-foot Speculum which Lord Rosse published in the Philosophical Transactions for 1840, he speaks of the possibility of one six feet in diameter being cast. It might at that time have been considered as little less than a chimera by those who were not sufficiently acquainted with the experiments that had been made in his Lordship's laboratory, and there were not wanting some who denied altogether the practicability of the design. Various reasons were given why the attempt should be a failure, and many calculations entered into to prove the little benefit to be derived even supposing a perfect casting were obtained—But fortunately others thought differently; the idea had no sooner occurred to Lord Rosse than he determined to put it to the test, and we may say, without flattery, that no absurdity was likely to occupy a mind like his. The attempt has been made, and the result is perfect success.

As yet we cannot say how far it may advance our knowledge of the celestial spheres, or help us to understand more fully the mechanism of the universe; but this at all events is certain, that be the advantage great or small, it is the last step that can be taken to enlarge our acquaintance with those distant bodies, and all that is ever likely to be brought before us will now be seen.

Lord Rosse having satisfied himself by experiments with lenses that the Refracting Telescope could not be much improved, turned his attention to Reflectors; and as the first object of experimenters had always been to increase the magnifying power and light by the construction of as large a mirror as possible, so was it to this point that his Lordship's attention was also directed.

Previous to his experiments, there had not been any instrument constructed, with the exception of Sir W. Herschell's, which had given an opportunity of sufficiently well enjoying the advantage of the Reflecting Telescope; and even of this it has been lately stated, that it possessed but little, if any, practical superiority over others of smaller size.

Since Newton manufactured his Specula until the present day, there have been several opinions both as to the metals to be employed in their construction, and the quantities in which they should be mixed—some have recommended various proportions of tin and copper; some have added arsenic; some silver; some antimony; and others, the three together—the general aim of all the operators being to increase the whiteness, and to diminish the porosity and brittleness of the compound; for the last named property has destroyed a much greater number of Specula than it has allowed to be completed—it is the “asses' bridge” over which few have been fortunate enough to travel.