he can hardly be said to be its inventor, as the principle had been promulgated by Mersenne in his "Cutoptrics," several years before Gregory wrote. The idea was unquestionably valuable, but it was even then, in 1663, inoperative; for although its author-if he must so be termed, though hardly, as has just been said, entitled to the appellation-came to London for the purpose, he could nowhere meet with an artist who could undertake the formation of such a mirror as he had designed; and the attention of men of science was once more earnestly directed to the improvement of the dioptric telescope.

Here again great difficulties had to be encountered; for, after Sir Isaac Newton's discovery of the refragibility of light, it was found that the aberration about the focus of a lens was many hundred times greater than could be accounted for by the form of a glass. As the aberration in a mirror was smaller, and without the chromatic confusion, and consequently much more distinct, Newton set himself to construct such a mirror. Accordingly, early in the year 1669, he obtained a composition of metals likely to suit his purpose, and with his own hands did the greater part of the work for grinding its surface to the form of a sphere. By the year 1677 he had completed two telescopes, an account of which, and the result of the use of them, he sent to the Royal Society, in whose journals it was published. This telescope had a magnifying power of thirtyeight, and the radius of the concave was thirteen inches.

About the same time Mr. Gregory succeeded in accomplishing the design which he had for so many years entertained; and M. Cassegrain, in France, also described the principles on which a reflecting telescope might be made.

Near this period, also, Dr. Hook was engaged in the improvement of the telescope; and in 1674 he produced the first reflecting instrument in which the great speculum was perforated, so that objects might be viewed by looking directly

at them, and submitted it to the Royal Society in the February of that year. About 1720, Dr. Bradley, professor of anatomy at Oxford, who had heretofore used, in most of his observations, the long focal instrument of Huygens, applied himself, in conjunction with a gentleman of the name of Molyneux, who resided at Kew, to the improvement of reflecting telescopes, especially with a view to reducing the inconvenient size in which they were made. They succeeded admirably; and having, in 1738, directed two London opticians, Messrs. Hearne and Scarlet, in their mode of construction, these artists were soon enabled to manufacture reflecting telescopes for general use.

Soon after this a maker at Edinburgh, Mr. James Short, was assiduously engaged in the endeavour to form specula, and from his investigations it was ascertained that glass had not sufficient steadiness to preserve a correct parabolic figure; but he succeeded, it is believed, so far as human eye could perceive, in obviating that defect, his telescopes allowing of larger apertures, and of course a better observation of the object.

The improvement of specula, during the whole of the eighteenth century, was sought by all earnest opticians; and, at last, Sir William Herschel, whose numerous discoveries have given him a right to the title of the greatest astronomer after Newton, gave to the reflecting telescope the greatest powers which it had ever up to that time attained. While laboriously engaged in obtaining, as a musician, a daily subsistence, Herschel occupied his leisure hours in the construction of telescopes, both of the Gregorian and of the Newtonian kind; and about 1783, being aided by the liberality of King George III., to whose notice he had been introduced through his discovery of the planet called the Georgium Sidus, he set to work to make a telescope of forty feet in focal length, after Newton's principles. He succeeded in fully accomplishing his

object in the year 1789; and the very night after its completion he discovered the remote orb which is the sixth satellite of Saturn.

This telescope possesses a magnifying power of 6,500 times, and has, in the hands of Sir William Herschel, and of his son, the present eminent astronomer, Sir John Herschel, proved immensely serviceable in the promotion of astronomical science, and in the improvement of those arts and professions to which it is made subservient.

We must not, however, conclude our description without making honourable mention of another name connected with the improvement of this useful instrument. After Sir William Herschel, came John Ramage, an Aberdeen merchant, who, as early as the year 1806, had made reflectors with specula six inches in diameter. These he improved upon, and, only four years after, produced an instrument with a focal length of eight feet, and a mirror that measured nine inches. Not yet satisfied, he ventured still farther, and from a focal length of twenty feet, with a specula thirteen and a half inches in diameter, he at length completed telescopes twenty-five feet long, with mirrors of fifteen inches. Although these reflecting telescopes showed the double stars very distinctly, yet in no instance did they aid in any new discovery; not even when Ramage had succeeded in making an instrument with a focal length of fifty-four feet, and a speculum twenty-one inches in diameter-a clear proof that the power of the reflecting telescope, as regarded discoveries, could not be carried beyond the improvements made by Herschel.

While the reflecting telescope was thus progressing towards its present state of perfection, the endeavour to diminish the fringe of colours which surrounded the appearance of objects when viewed through dioptric instruments did not cease. An improvement made by Mr. Chester Hall, in 1729, greatly facilitated the attainment of a clear image through the eye

glass. It will be remembered that he endeavoured to accomplish that object by using lenses of different kinds of glass; and his idea was further carried out by the celebrated optical instrument-maker and philosopher Mr. Dolland, about thirty years afterwards. In consequence of strictures made upon some observations which he published in the "Philosophical Transactions," on a proposition of Euler's to use hollow spherical segments of glass with water between them, to diminish the aberration in telescopes, Mr. Dolland was led to make experiments on wedges of different kinds of glass to ascertain the

various degrees of refragibility which they occasioned. He ultimately discovered that 'by using a convex lens of crown glass, and a concave lens of flint glass, the different coloured rays in each pencil of light, after refraction through both, fell upon the eye nearly colourless. For this improvement he was presented with the Copleian medal by the Royal Society; and a few years afterwards, in 1765, his son, Mr. Peter Dolland, made a still further improvement by diminishing the aberration occasioned by the spherical form of the glass. He placed a concave lens of flint glass between two convex lenses of crown glass, an arrangement which almost altogether did away with the fringed coloration of the image, and gave the still further advantage of a large aperture for the observation of the object when the focal strength of the instrument is short.

Various improvements were afterwards made by Mr. Ramsden and others, chiefly with a view to destroy the aberration through the union of spheres of different kinds of glass. But the greatest triumph that ever was achieved in the conjunction of philosophical acumen and mechanical skill, has been accomplished in our own day, and through the agency of a nobleman whose name will live as long as the human faculties shall be exercised in observing the magnificent wonders of the central universe. This is the monster telescope constructed by Lord Rosse.

To give a description of this wonderful instrument would, within our limits, be an utter futility, for it deserves a separate and entire volume; but the following account, first sent to the "Times," by Sir James South, will convey some idea of its power and magnitude.

"The diameter of the large speculum is 6 feet, its thickness 5 inches, its weight 3 tons, and its composition 126 parts of copper to 57 parts of tin; its focal length is 54 feet—the tube is of deal; its lower part, that in which the speculum is placed, is a cube of 8 feet; the circular part of the tube is, at its centre,