VOL. LXXIV.] PHILOSOPHICAL TRANSACTIONS. 475 



light a 20th part, while the velocity of the light of the other, which was sup- 

 posed to revolve about it as a satellite, for want of sufficient magnitude in the 

 body whence it was emitted, should suffer no sensible diminution at all. Placing 

 then the line, in which the two faces of the prism would intersect each other, at 

 right angles to a line joining the two stars ; if the thinner part of the prism lay 

 towards the same point of the heavens with the central star, whose light would 

 be most turned out of its way, the apparent distance of the stars would be in- 

 creased 2" 53'", and consequently become 3" 53"', instead of l^only, the appa- 

 rent distance supposed above in art. 21. On the contrary, if the prism should 

 be turned half way round, and its thinner part lie towards the same point of the 

 heavens with the revolving star, their distance must be diminished by a like quan- 

 tity, and the centraj star therefore would appear \" 53'" distant from the other 

 on the opposite side of it, having been removed from its place near 3 times the 

 whole distance between them. 



34. As a prism might be made use of for this purpose, which should have a 

 much larger refracting angle than that we have proposed, especially if it was con- 

 structed in the achromatic way, according to Mr. Dollond's principles, not only 

 such a diminution, as 1 part in 20, might be made still more distinguishable ; 

 but we might probably be able to discover considerably less diminutions in the 

 velocity of light, as perhaps a 100th, a 200th, a 500th, or even a 1000th part 

 of the whole, which, according to what has been said above, would be occa- 

 sioned by spheres, whose diameters should be to that of the sun, provided they 

 were of the same density, in the several proportions nearly of 70, 50, 30, and 

 22 to 1 respectively. 



35. If such a diminution of the velocity of light, as that above supposed, 

 should be found really to take place, in consequence of its gravitation towards 

 the bodies, whence it is emitted, and there should be several of the fixed stars 

 large enough to make it sufficiently sensible, a set of observations on this subject 

 might probably give us some considerable information with regard to many cir- 

 cumstances of that part of the universe, which is visible to us. The quantity of 

 matter contained in many of the fixed stars might hence be judged of, with a 

 great degree of probability, within some moderate limits; for though the exact 

 quantity must still depend on their density, yet we must suppose the density 

 most enormously different from that of the sun, and more so indeed than one 

 can easily conceive to take place in fact, to make the error of the supposed quan- 

 tity of matter very wide of the truth, since the density, as has been shown 

 above in art. 1 1 and 12, which is necessary to produce the same diminution in 

 the velocity of light, emitted from different bodies, is as the square of the quan- 

 tity of matter contained in those bodies inversely. 



.36. But though we might possibly hence form some reasonable guess at the 



3 P 2 



