VOL. LXIV.] PHILOSOPHICAL TRANSACTIONS. 49 1 



take to traverse the whole disc, is nearly equal to the time that they are hid by 

 being on the opposite surface. It is plain, that the time of their appearing on 

 the disc must be some small matter shorter than that of their being hid behind 

 it, on account of our not seeing a complete hemisphere of the sun. But fur- 

 ther, it must now be considered, that when a spot just enters the disc, the part 

 which is first visible, is the farthest umbra, by which time the spot has really 

 advanced a whole diameter of itself on the disc. And again, when the same spot 

 goes off the disc, it is evident, that the part which is last visible, is then the 

 farthest umbra, on which account the continuance of the spot on the disc will be 

 shortened by an interval of time, which corresponds nearly to the whole breadth 

 of it. This, as well as the other appearances, described in the first part of this 

 paper, concerning the change of the umbra and figure of the nucleus, when 

 spots approach the limb, are all well illustrated, by making, in a sphere, an 

 excavation similar to what we have described, the bottom of which may be 

 painted black to represent the nucleus, and the sloping sides shaded, if the sphere 

 be of a light colour. 



According to the view of things given in the foregoing queries, there would 

 seem to be something very extraordinary in the dark and unignited state of the 

 great internal globe of the sun. Does not this seem to indicate, that the lumi- 

 nous matter which encompasses it derives not its splendour from any intensity of 

 heat ? For if this were the case, would not the parts underneath, which would 

 be perpetually in contact with that glowing matter, be heated to such a degree, 

 as to become luminous and bright.'' At the same time it must be confessed, that 

 though the internal globe was in reality much ignited, yet when any part of it, 

 forming the nucleus of a spot, is exposed to our view, and is seen in competition 

 with a substance of such amazing splenclor, it is no wonder that an inferior de- 

 gree of light should in these circumstances be unperceivable. But from the 

 nature of the thing, does there seem any necessity for thinking, that there pre- 

 vails there any such raging and fermenting heat, as many have imagined? It is 

 proper here to attend to the distinction between this shining matter of the sun, 

 and the rays of light which proceed from it. It may perhaps be thought, that 

 the re-action of the rays on the matter, at their emission, may be productive of 

 a violent degree of heat. But whoever would urge this argument, in favour of 

 the sun being intensely heated, as arising from the nature of the thing, ought 

 to consider, that all polished bodies are less and less disposed to be heated, by 

 the action of the rays of light, in proportion as their surfaces are more polished, 

 and as their powers of reflection are brought to a greater degree of perfection. 

 And is there not a strong analogy between the re-action of light on matter, in 

 cases where it is reflected, and in cases where it is emitted? 



It may perhaps be expected, that in this paper, mention should be made of the 



3 B'2 



