CONNECTION WITH PLANETARY CONFIGURATIONS. 503 



variable stars ; but before doing so, I would remark, that when the sun's disc has 

 many spots on it, we probably derive less light from it than when it is free from 

 spots. This is not altogether self-evident, for each spot is accompanied by 

 faculse, and it has been observed by Mr Warken De la Rue and others, that these 

 faculse, when near the sun's limb, are much brighter than the neighbouring surface 

 of the disc, but when in the centre they are not sensibly brighter than the neigh- 

 bouring surface. When, therefore, a spot occurs near the limb, the loss of light 

 which it implies may possibly be replaced by the faculse which accompany it ; but 

 in this position, both spot and faculse are much foreshortened, and present but a 

 small field of view. On the other hand, the same spot, when near the centre of the 

 disc, fills a much larger field of view, and the loss of light which this implies is 

 not made up by any superior brightness in the accompanying faculse beyond that 

 of the neighbouring disc. On the whole, therefore, we have a loss of light occa- 

 sioned by spots. Against this it may perhaps be argued, that when the sun's 

 disc is full of spots the general luminosity is greater than during those periods 

 when there are comparatively few spots ; but we have no proof for such an 

 assertion. Assuming, therefore, that a disc full of spots is deficient in lumino- 

 sity, let us now turn to the phenomena of variability presented in other stellar 

 systems. Without entering into details, it will be sufficient to mention the hypo- 

 thesis which many astronomers believe in as serving well to represent these pheno- 

 mena, I allude to that of rotation on an axis, where it is supposed that the body 

 of a star is from some cause not equally luminous in every part of its surface. It 

 is in the last clause of this sentence that the deficiency of such an hypothesis as a 

 physical, and not a mere formal explanation, consists : for it is exceedingly diffi- 

 cult to conceive a body, one part of which is permanently luminous, and another 

 part permanently dark. If, however, we conceive of a variable star as a sun 

 round which a planet of some magnitude revolves at a small distance from its 

 primary, and adopt the law which probably obtains in our own system, we shall 

 have a state of things phenomenally equivalent to a body partly dark and partly 

 bright. For as the planet moves round its primary, that portion of the disc of 

 the latter next the planet will be brighter than that which is more remote, and 

 this appearance will move round as the planet itself moves. 



It still remains to discuss the case of a planet with a very elliptical orbit. 

 Such a body would for a very long period of time be far removed from its 

 primary, and for a short period of time extremely near it. We might therefore 

 expect, according to the above law, a long period of comparative darkness in the 

 primary, followed by a short period of brilliancy, corresponding to the perihelion 

 of the planet. Now, this is precisely the appearance presented by temporary 

 stars. These bodies emerge from comparative obscurity, become suddenly bril- 

 liant, and thereafter very soon begin to fade, darkness being their normal state, 

 while their brilliancy is short-lived and exceptional. 



VOL. XXIII. PART in. 6 u 



