VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 473 



It is also not very improbable, that there is some difference from that of the sun, 

 in the constitution of those stars, which have sometimes appeared and sometimes 

 disappeared, of which that in the constellation of Cassiopeia is a notable instance. 

 And if those conjectures are well founded which have been formed by some phi- 

 losophers concerning stars of these kinds, that they are not wholly luminous, or 

 at least not constantly so, but that all, or by far the greatest part of their sur- 

 faces is subject to considerable changes, sometimes becoming luminous, and at 

 other times being extinguished ; it is among the stars of this sort, that we are 

 most likely to meet with instances of a sensible apparent diameter, their light 

 being much more likely not to be so great in proportion as that of the sun, 

 which, if removed to 400,000 times his present distance, would still appear, I ap- 

 prehend, as bright as Sirius, as I have observed above ; whereas it is hardly to 

 be expected, with any telescopes whatever, that we should ever be able to distin- 

 guish a well defined disc of any body of the same size with the sun at much 

 more than 10,000 times his distance. 



28. Hence the greatest distance at which it would be possible to distinguish any 

 sensible apparent diameter of a body, as dense as the sun, cannot well greatly 

 exceed 5 million times the distance of the sun ; for if the diameter of such a 

 body was not less than 500 times that of the sun, its light, as has been shown 

 above, in art. l6, could never arrive at us. 



29. If there should really exist in nature any bodies, whose density is not less 

 than that of the sun, and whose diameters are more than 500 times the diame- 

 ter of the sun, since their light could not arrive at us ; or if there should exist 

 any other bodies of a somewhat smaller size, which are not naturally luminous; 

 of the existence of bodies under either of these circumstances, we could have no 

 information from sight ; yet, if any other luminous bodies should happen to re- 

 volve about them, we might still perhaps from the motions of these revolving 

 bodies, infer the existence of the central ones with some degree of probability, 

 as this might afford a clue to some of the apparent irregularities of the revolving 

 bodies, which would not be easily explicable on any other hypothesis ; but as the 

 consequences of such a supposition are very obvious, and the consideration of 

 them somewhat beside my present purpose, I shall not prosecute them any 

 further. 



30. The diminution of the velocity of light, in case it should be found to 

 take place in any of the fixed stars, is the principal phenomenon whence it is 

 proposed to discover their distance, &c. Now the means by which we may find 

 what this diminution amounts to, seem to be supplied by the difference which 

 would be occasioned in consequence of it, in the refrangibility of the light, 

 whose velocity should be so diminished. For let us suppose with Sir Isaac New- 

 ton (Optics, prop. 6, paragr. 4 and 5) that the refraction of light is occasioned 



vol. xv. 3 P 



