is better adapted to many purposes in astronomy than that of Gregory, although for common purposes Gregory's may be considered most convenient.

Many inconveniences attended the construction and execution of reflecting telescopes. When made, they were liable to tarnish, and to change their figure, an error in which is of much greater consequence than in refractors. Thus much fewer advantages were derived from reflecting telescopes than had been expected. And the improvement of telescopes seemed at a stand, when, in the year 1757, a discovery of Mr. Dolland, an optician in London, gave hopes of improving them far beyond what had been hitherto done. He discovered that by a combination of lenses of flint glass and crown glass, he could form an image free from colour. This enabled him to make telescopes, admitting of high magnifying powers, of a very convenient length. These telescopes, called achromatic, are now in common use, and fitted to those astronomical instruments by which angles are measured. Expectations were formed of being able to increase the breadth of the object glasses, to admit of very high magnifying powers, without lengthening the telescope so as to be inconvenient; but this was prevented by the nature of flint glass. This cannot be obtained fit for the purpose of telescopes, except in small portions of surface. It does not appear that we can do more by achromatic telescopes, than astronomers at the end of the 17th century did by telescopes without tubes, if so much; and achromatic telescopes, although an invaluable improvement by reducing the length of telescopes, have not discovered to us more in the heavens than had been seen a century before.

21. Under these circumstances, the very ingenious and indefatigable Dr. Herschel set himself to improve reflecting telescopes, in which he has been highly successful. His reflectors are of the Newtonian kind. After repeated attempts he suc

ceeded in making one 20 feet long and 18 inches aperture. The great breadth of the aperture increased so much the brightness of the image, that he was enabled, with great convenience, to use very high magnifying powers. At last he attempted and executed one 40 feet in length and of 4 feet aperture. A most surprising performance, when the labour and difficulty of casting and polishing the metal speculum, the obstacles he had to contend with in the weight, and in the apparatus for moving it, are considered. A full account of this telescope, by Dr. Herschel himself, is given in the Phil. Trans. for 1795.

The discoveries of Dr. Herschel will be mentioned in their places. In the mean time it may be remarked, in order to form some idea of the effect of telescopes, when applied to the celestial bodies, that the reflector of the 40 feet telescope forms an image of the ring of Saturn, about of an inch in diameter ; we are enabled to magnify this by the eye glass, in the same

manner as we can magnify an object by a common microscope.

of an inch in breadth

22. The appearance of the stars seen in a telescope, is very different from that of the planets. The latter are magnified, and shew a visible disc. The stars appear with an increased lustre : but with no disc. Some of the brighter fixed stars appear most beautiful objects, from the vivid light they exhibit. Dr. Herschel tells us, that the brightness of the fixed stars of the first magnitude, when seen in his largest telescope, is too great for the eye to bear. When the star Sirius was about to enter the telescope, the light was equal to that on the approach of sun rise, and upon entering the telescope, the star appeared in all the splendour of the rising sun, so that it was impossible to behold it without pain to the eye.

The apparent diameter of a fixed star is only a deception arising from the imperfections of the telescope. The brighter

stars appear sometimes in bad telescopes to subtend an angle of several seconds, and this has led astronomers into mistakes respecting their apparent diameters. The more perfect the telescope, the less this irradiation of light. We know certainly that some of the brighter fixed stars do not subtend an angle of 1", from the circumstance of their instantly disappearing, on the approach of the dark edge of the moon. Dr. Herschel attempted to measure the diameter of a Lyræ, and imagined it to be about of a second.

23. Although the superior light of the sun effaces that of the stars, yet by the assistance of telescopes we can observe the brighter stars at any time of the day. The aperture of the telescope collects the light of the star, so that the light received by the eye is greater than when the eye is unassisted. The darkness in the tube of the telescope also in some measure assists.a

The most inferior telescope will discover stars that escape the unassisted sight. By the telescope we discover that the milky way, and some of the nebulæ above-mentioned, consist of very numerous small stars. Others, even in the best telescopes,

a It appears by the principles of optics, that when an object is seen through a telescope, the density of the light on the retina must be always less than when the object is seen by the naked eye; but the quantity of light in the whole image may be much greater in the former case than in the latter. And it is certain that our power of seeing the object with distinctness, depends on the quantity of light in the whole image. Dr. Herschel, in a valuable paper in the Phil. Trans. 1800, part I. on the power of penetrating into space, uses the terms absolute brightness and intrinsic brightness, the former to distinguish the whole quantity of light in the image on the retina, and the latter to distinguish its density. He gives an instance in which the absolute brightness was increased 1500 times in a telescope, and the intrinsic brightness was less than to the naked eye in the proportion of 3 to 7.

appear still as small luminous clouds. There is a very remarkable one in the constellation of Orion, which the best telescopes shew as a spot uniformly bright. It is a singular and beautiful phenomenon. So great is the number of telescopic stars in some parts of the milky way, that Dr. Herschel observed 588 stars in his telescope at the same time, and they continued equally numerous for a quarter of an hour. In a space about 10 degrees long, and 2 degrees wide, he computed there were 258000 stars. Phil. Trans. 1795.

24. The most ancient catalogue of the fixed stars is that of Hipparchus, who observed at Alexandria about 150 B. C. His catalogue consists of 1022 stars. Although several celebrated astronomers, as Tycho Brahe, &c. employed themselves in more accurately observing the places of the fixed stars, yet the number was not much increased till the time of Flamstead, whose catalogue, entiled the British Catalogue, appeared in 1725. It contains about 3000 stars visible to the naked eye, and was the result of nearly 40 years labour. Later astronomers have observed, with greater accuracy, the places of some of these stars, particularly of those in and near the zodiac; and very recently, M. Piazzi, of the Observatory at Palermo, in Sicily, has recompleted the whole catalogue. In 1802, M. Delalande published a work entitled Histoire céleste Française, in which are observations of 50000 stars, viz. of stars of the 6th magnitude not observed by Flamstead, and of telescopic stars of the 7th, 8th, and 9th magnitudes. They were mostly observed by his nephew, M. Lefrançais Delalande, and furnish a lasting monument of his patience and industry. Great as is this number of stars of the above magnitudes, it would not be difficult to increase it considerably.

25. Some stars appearing single to the naked eye, when examined with a telescope appear double or treble, that is, consisting of two or three stars very close together: such are Castor,

a Herculis, the Pole Star, &c. Seven hundred, not noticed before, have been observed by Dr. Herschel. They are particularly useful for trying and comparing the goodness of telescopes, because if the telescope do not give a well defined image, these stars will appear as one. In viewing these double stars a singular phenomenon discovers itself, first noticed by Dr. Herschel; some of the double stars are of different colours, which, as the images are so near each other, cannot arise from any defect in the telescope. a Herculis is double, the larger red, the smaller blue; & Lyræ is composed of four stars, three white and one red; y Andromedæ is double, the larger reddish white, the smaller a fine sky blue. Some single stars evidently differ in their colour. Aldebaran is red, Sirius brilliant white.

From observations at different periods it appears considerable changes have taken place among the fixed stars. Stars have disappeared, and new ones have appeared. The most remarkable new star recorded in history, was that which appeared in 1572, in the chair of Cassiopea. It was for a time brighter than Venus, and then seen at mid-day: it gradually diminished in lustre, and after sixteen months disappeared. That the circumstances of this star were faithfully recorded we can have no doubt, since many different astronomers of eminence saw and described it. Cornelius Gemma viewed that part of the heavens on November 8, 1572, the sky being very clear, and saw it not. The next night it appeared with a splendor exceeding all the fixed stars, and scarcely less bright than Venus. Its colour was at first white and splendid, afterwards yellow, and in March, 1573, red and fiery like Mars or Aldebaran, in May of a pale livid colour, and then became fainter and fainter till it vanished.

Another new star, little less remarkable, appeared in October, 1604. It exceeded every fixed star in brightness, and