1 66 Dr. Wilson’s Anfwer to the 
testimony to the exterior boundary of the umbra being always 
well defined, and to the umbra itfelf being lefs and leSs Shady 
the nearer it comes to the nucleus. Now it may be aSked, 
how this could pofiibly be according to M. de la lande’s 
theory ? It the umbra be occasioned by our feeing parts of the 
opaque rock, which lie a little under the furface of the igneous 
matter, Should it not always be darkeSt next the nucleus, and 
from the nucleus -outward Should it not wax more and more 
bright, and at laffc lo.fe itfelf in the general luStre of the fun’s 
furface, and not terminate all at once at the darkeSl Shade, as in 
fadt it does ? Thefe few incongruities, which meet us as it were 
in the very threfhold of the theory, are fo very palpable, that of 
themfelves they raile unfurmountable doubts. For, generally 
Speaking, the umbra immediately contiguous to the nucleus, in- 
stead of being very dark, as it ought to be, from our feeing the 
immerfed parts of the opaque rock through a thin Stratum of 
the igneous matter, is on the contrary very nearly of the fame 
Splendour as .the external furface. 
Concerning the nucleus, or that part of the opaque rock 
which Stands above the furface of the fun, M. de la lands 
produces no optical arguments in fupport of this third dimen- 
sion or height. Neither does he fay any thing particular as to 
the degree of elevation above the furface. But from what has 
been already hinted in the courfe of this paper, it appears, that 
; if this were any thing fenfible, it ought to be difcovered by 
phenomena very opposite to thofe which we have found to he So 
Again, a flux and reflux of the Igneous matter fo considera- 
ble as fometimes to produce a great number of fpots all over 
the middle zone, might affedt the apparent diameter of the 
fun, making that which paSTes through his equator lefs than 
the 
