370 PHILOSOPHICAL TRANSACTIONS. [ANNO 1783. 



carry with thein a perfect conviction, are those grounded on the principles of 

 optical projection. If, for example, by far the greater number of them be ex- 

 cavations, some thousands of miles deep, certain changes of the umbra would be 

 observable when near the limb. Were they very shallow, or quite superficial, 

 both sides of the umbra would as to sense contract alike in their progress toward 

 the limb. Again, if the nucleus extended much above the common level while 

 the surrounding umbra was superficial, we should behold manifest indications of 

 this by such an opaque body, when seen very obliquely, being projected across 

 the farthest side of the umbra, and by hiding the whole or part of it before the 

 time it would otherwise disappear. According to this or that condition of the 

 spot, such things must infallibly obtain by the known laws of vision ; and hence 

 arguments resting on such principles may be denominated optical ones. On the 

 other hand, when spots are contemplated near the middle of the disk, a great 

 variety of changes are observed in them, which depend not upon position, but 

 on certain physical causes producing real alterations in their form and dimensions. 

 It is plain, that arguments derived from the consideration of such changes, and 

 which, on that account, may be called physical arguments, can assist us but little 

 in investigating their 3d dimensions ; and, from the nature of the thing, must be 

 liable to great uncertainty. 



Again : but whatever be their defects, no doubts ought to arise, on such 

 grounds, of the spots being themselves what direct observation declares them, 

 namely, excavations in the sun. Whether their first production and subsequent 

 numberless changes depend on the eructation of elastic vapour from below, or on 

 eddies or whirlpools commencing at the surface, or on the dissolving of the 

 luminous matter in the solar atmosphere, as clouds are melted and again given 

 out by our air ; or, if the reader pleases, on the annihilation and reproduction 

 of parts of this resplendent covering ; is left for theory to guess at. Though, 

 however, many difficulties should occur in an attempt of this kind, it would 

 certainly be unreasonable on that account to call in question the 3d dimension of 

 the spots, as previously determined by arguments which are liable to no fallacy, 

 and which are unconnected with every kind of theoretical reasoning. 



As I conceive it, however, of some importance to have the distinction above 

 treated of perfectly understood in future, I now purposely avoid entering on 

 any theoretical ground whatever. My wish therefore is, that the author of the 

 Memoire may acquit me of every thing not perfectly respectful, though I do not 

 follow him through that train of objection founded on vague and incompetent 

 physical arguments, which is to be met with in p. 511, &c. By further con- 

 sidering the particulars hinted at in p. 21 and 29, of my paper, several difficulties, 

 perhaps, may be removed ; but we forbear any illustration of this kind, chiefly to 

 evince how little we concern ourselves whether those who do not like the princi- 



