53 
of the Sun and fixed Stars. 
the shining elastic fluid, when, by some agitation or other 
cause, it is not of the usual height ; and this opinion is much 
strengthened by the return of some remarkable spots, which 
served Cassini to ascertain the period of the sun's rotation. 
A very high country, or chain of mountains, may oftener be- 
come visible, by the removal of the obstructing fluid, than the 
lower regions, on account of its not being so deeply covered 
with it. 
In the year 1791, I examined a large spot in the sun, and 
found it evidently depressed below the level of the surface ; 
about the dark part was a broad margin, or plane of consi- 
derable extent, less bright than the sun, and also lower than 
its surface. This plane seemed to rise, with shelving sides, up 
to the place where it joined the level of the surface. 
In confirmation of these appearances, I carefully remarked 
that the disc of the sun was visibly convex ; and the reason of 
my attention to this particular, was my being already long ac- 
quainted with a certain optical deception, that takes place now 
and then when we view the moon ; which is, that all the ele- 
vated spots on its surface will seem to be cavities, and all ca- 
vities will assume the shape of mountains. But then, at the 
same time the moon, instead of having the convex appearance 
of a globe, will seem to be a large concave portion of an hol- 
low sphere. As soon as, by the force of imagination, you drive 
away the fallacious appearance of a concave moon, you restore 
the mountains to their protuberance, and sink the cavities again 
below the level of the surface. Now, when I saw the spot 
lower than the shining matter of the sun, and an extended 
plane, also depressed, with shelving sides rising up to the 
level, I also found that the sun was convex, and appeared in 
