these thoughts I was forced from Cambridge by the intervening plague, and it was more than two years before I proceeded further.’ It was towards the end of 1668, or in the beginning of the following year, when Newton, being obliged to have recourse to reflectors, and not relying on any artificer for making the specula, set about the work himself, and early in the year 1672, completed two small reflecting telescopes. In these he ground the great speculum into a spherical concave, although he approved of the parabolic form, but found himself unable to accomplish it. These telescopes were of a construction somewhat different from what Gregory had suggested, and though only 6 inches long, were considered as equal to a 6 feet common refracting telescope. It is not a little singular, however, that we hear no more about the construction of reflectors till more than half a century afterwards. It was not till the year 1723, that any reflectors were known to have been made, adapted to celestial observations. In that year, Mr. Hadley, the inventor of the reflecting quadrant, which goes by his name, published in No. 376 of the Philosophical Transactions, an account of a large reflector on Newton’s plan, which he had just then constructed, the performance of which left no room to doubt that this invention would remain any longer in obscurity. The large speculum of this instrument was 62-5/8 inches focal distance and 5 inches diameter, was furnished with magnifying powers of from 190 to 230 times, and equalled in performance the famous aerial telescope of Huygens of 123 feet in length.[25] Since this period, the reflecting telescope has been in general use among astronomers in most countries of Europe, and has received numerous improvements, under the direction of Short, Mudge, Edwards and Herschel--the last of whom constructed reflectors of 7, 10, 20, and even 40 feet in focal length, which far surpassed, in brightness and magnifying power, all the instruments of this description, which had previously been attempted. I shall now proceed to give a brief sketch of the nature of a reflecting telescope, and the different forms in which they have been proposed to be constructed.