naked eye, appear as a kind of fine white cloud. The number of nebulæ afcertained before the time of Herschel, are about 103." He has, however, given us a catalogue of 1000 new nebulæ; many of the nebulæ are refolvable by the telescope, into clusters of small stars; and it is found, that telescopes of greater force refolve those nebulæ into stars, which appear as white clouds in inftruments of lefs force. Hence then there is reason to conclude, that they all confift of clufters, or prodigious aggregates of stars. Herschel fays, that in the most crouded part of the milky way, he had fields of view, that contained no less than 588 ftars, and these were continued for many minutes, fo that in one quarter of an hour there passed no less than 116,000 ftars through the field of view. of his telescope. The blended luftre of these luminous points, is the cause of that light and white appearance, from whence this space has been denominated.

Dr.

Dr. Herschel endeavours to make it appear, both from obfervation and well grounded conjecture, 1. That the ftarry heaven is replete with nebulæ, or systems of ftars of various figures; and that the milky way is that particular nebula in which our fun is placed. 2. That each nebula is compofed of a prodigious number of funs,

or

or bodies, that shine by their own native splendor. 3. That each individual sun is destined to give light to hundreds of worlds that revolve about it, but which can no more be feen by us, on account of their great distance, than the folar planets can be seen from the fixed ftars. Nor is it necessary that in this explored and unexplorable abyfs of space, the planets fhould be of the fame magnitude as those that belong to our fyftem; for it is not improbable, but that planetary bodies may be discovered among the double and triple stars.

Dr. Herschel mentions fome other heavenly bodies difcovered by him, which, from the fingularity of their appearance, he finds it difficult to know how to clafs: he has termed them planetary nebulæ, for he can hardly fuppofe them to be nebulæ; their light is fo uniform, as well as vivid; their diameter fo fmall, and welldefined; if nebulæ, they muft confift of stars that are compreffed and accumulated in the highest degree.

OF THE TELESCOPIC APPEARANCE OF THE

PLANETS.

By means of the telescope, we are enabled, in fome measure, to afcend into the celeflial region, and view the fun, moon, and flars, as they would appear to us if they were brought fo many times nearer to us as the telescope magnifies; the light proceeding from the luminary we are looking at, being diminished in the fame proportion.

The telescope is one of thofe discoveries, of which no idea could have been formed, previous to the period in which the Supreme Being was pleased to unveil to the human mind fome of the myfterious powers of glafs: the importance of this discovery, and the extent to which it may be carried, ftill lie hid among the fecrets of infinite wisdom.

When we look at the fun through a telescope even of moderate power, the eye being defended by a piece of coloured or smoked glass, nay, even by the naked eye, when guarded in the fame manner, we discover on his furface many black, or rather lefs bright fpots, of various fizes and fhapes. Sometimes thefe fpots will vanith in a very short time after their first appearance; fome

times they travel over his whole disk, or visible furface, from weft to eaft, when they disappear, and in twelve or thirteen days they appear again, fo as to be known, by their magnitude and figure, to be those that had difappeared before. Thofe, however, which are of the longeft continuance, do not appear to have much folidity of confiftence, for in a little time they also vanish, and become bright like the reft of the furface.

These spots are more frequent at fome periods than at others; in fome years, the fun's disk has for many months been perfectly free from them; in others, he has for months been more or less obfcured by spots: the most remarkable phænomena of these spots, as obferved by Scheiner and Hevelius, are as follow: 1. Every spot, which has a nucleus, or dark part, hath also an umbra, or fainter fhade, furrounding it. 2. The boundary betwixt the nucleus and umbra is always distinct and well-defined. 3. The increase of a fpot is gradual, the breadth of the nucleus and umbra dilating at the fame time. 4. In like manner, the decrease of a spot is gradual, the breadth of the nucleus and umbra diminishing. at the fame time. 5. The exterior boundary of the umbra never confifts of fharp angles, but is always curvilinear, how irregular foever the outfide

of

of the nucleus may be. 6. The nucleus of a fpot, whilft on the decrease, often changes it's figure, by the umbra incroaching irregularly upon it, infomuch that in a small space of time new incroachments are difcernible, whereby the boundary between the nucleus and the umbra is perpetually varying. 7. It often happens, that by these incroachments the nucleus of a spot is divided into two or more nuclei. 8. The nuclei of the spots vanish before the umbra. 9. Small umbræ are often feen without nuclei. 10. A large umbra is feldom feen without a nucleus in the middle of it. 11. When a spot, which confifted of a nucleus and an umbra, is about to difappear, if it be not fucceeded by a facula, or fpot, brighter than the reft of the disk, the place it occupied is in a very little time not to be perceived.

In the Philof. Tranf. vol. Ixiv. the reader will find several curious obfervations on these spots, by Profeffor Wilson, and the Rev. Mr. Wolaston. The latter gentleman fays, he once faw, with a twelve-inch reflector, a fpot burft to pieces, while he was looking at the fun; the appearance was to him as that of a piece of ice, when dashed on a frozen pond, which breaks to pieces, and flides in various directions.