the fame place. But in the mean while it deferves our confideration, that if the fpot be quite on the furface of the Sun itfelf, it will only be visible in its progrefs from (d) to (d); and that at (d), on each fide, it will be feen as it were almoft edgewife, and appear narrower; and will be quite invifible on the apparent luminous difk of the Sun, near to the extreme limits (S) and (S).

9. And this fort of mode of appearing, and of difappearing, we find to be actually the real cafe, as to the appearance of almost all the Sun's fpots. They appear narrow when they first enter on the difk ;--but are not ufually feen entering at first on the very outward limb of the Sun. They appear fulleft, and broadeft, on the middle of the Sun's difk-they again appear narrow as they move forward towards the contrary limb;but are again ufually loft, before they reach the extreme limb of the apparent difk.

And this uniformity of appearance, and fome further refearches, in confequence of it, induced Dr. Herfchell, at laft, long fince my

* In his Obfervations on the Nature and Construction of the Sun and Fixed Stars, in the Philof. Tranf. for 1795, p. 46.

Morfels

Morfels of Criticifm were first printed, to come, in effect, to fomewhat the fame conclufion with me:-that the real body of the Sun is lefs than its apparent diameter ;—and that we never difcern the real body of the Sun itself, except when we behold its spots ;and that the Sun is inhabited as well as our earth;-and is not neceffarily fubject to burning heat;-and that there is, in reality, no violent elementary beat exifting in the rays of the Sun themfelves effentially.

In the first place, he difcovered, by means of repeated obfervations, that the Sun has a very extenfive atmosphere*; and that what appears to us as the disk of the Sun, confifts not merely of its real body, but of an atmosphere surrounding it, in appearance at leaft between two and three thousand miles in height; and that we rarely fee any thing more of the Sun, than its fhining atmosphere; its real body never being vifible, but when we fee fuch parts of it as appear as dark fpots.

He difcovered alfo, that this atmosphere of the Sun was very denfe; infomuch that the

Phil. Tranf. for 1795, p. 50. 59.

† Ibid. p. 62.

whole

whole difk fometimes appeared, in confequence of the vapours floating in its atmofphere, as covered with roughness, like the coat

of an orange*; and that, befides this, there were in various parts many apparent bright elevations, (or denfer vapour emerging apparently above the reft,) which he calls luminous facula;--and which feemed to confift of more bright and fhining matter than the reft.

In the next place, he made many repeated obfervations, more than fufficient to convince him, that the black fpots were all greatly depreffed below the furface of the Sun's luminous difkt; and that, in truth, when we behold

* Phil. Tranf. for 1795, p. 56, 57.

Whether the spots on the Sun are occafioned, either by any merely temporary check, or obstruction to the power of fending forth rays, and by the confequent temporary failing of emitted light;—or by means of any thick cloud of any kind of vapour, near to the body of the Sun, covering fuch part for a time, and obstructing the progrefs of the rays of emitted light till they are diffipated ;-in either cafe, they must produce exactly the effect on our eyes, when viewing the Sun, and precifely upon the fame optical principle, that is defcribed in the preceding and following pages.

Both caufes may perhaps fometimes concur; and fome of the appearances of fpots may even lead us to conclude, that they actually do fo; and that fometimes fuch clouds

are