454 On the Origin of the Light of the Sun and Stars. 



from the sun. Furthermore, an examination of the sun- 

 pictures taken by the Kew heliograph, seems to indicate the 

 following law. Any portion of the sun's disc which, owing to 

 his rotation, recedes from the neighbourhood of Venus, acquires 

 a tendency to break out into spots, and as it approaches Venus it 

 acquires a tendency to be free from spots. On the whole, there- 

 fore, we are perhaps entitled to conclude that, in our own system^ 

 the approach of a planet to the sun is favourable to luminosity, 

 and especially in that portion of the sun which is next the planet. 

 A confirmation of this law is found in the readiness with which 

 it may be adapted to other systems. Let us take variable stars. 

 The hypothesis which without being physically probable gives 

 yet the best formal explanation of the phenomena there pre- 

 sented, is that which assumes rotation on an axis, while it is 

 supposed that the body of a star is from some cause not equally 

 luminous in every part of its surface. Now if, instead of this, 

 we suppose such a star to have a large planet revolving round 

 it at a small distance, then, according to our hypothesis, that 

 portion of the star which is near the planet will be more lumi- 

 nous than that which is more remote, and this state of things 

 will revolve round as the planet itself revolves, presenting to a 

 distant spectator an appearance of variation with a period equal 

 to that of the planet. Let us now suppose the planet to have a 

 very elliptical orbit, then for a long period of time it will be at 

 a distance from its primary, while for a comparatively short 

 period it will be very near. We should, therefore, expect a 

 long period of darkness, and a comparatively short one of 

 intense light — precisely what we have in temporary stars. 

 Again, we have seen that in many binary systems there 

 is a change of magnitude, and that perhaps both members 

 change at the same time and in the same direction — a result in 

 favour of our hypothesis ; but it is to be regretted that we have 

 not yet sufficient data for determining if the brightness is 

 greatest when. both members are nearest together. Perhaps it 

 may now be asked, If the sun have not a large store of heat in 

 himself, but is fed from moment to moment, have we any 

 guarantee for the continuance of his light, or for its steadiness, 

 which is almost of equal importance to our well-being? We 

 reply, that our sun is not tho member of a binary system of 

 small period and large elliptieity, which might give him a 

 variable brightness, nor is he surrounded by planets that now 

 press near to him and anon recede fco a great distance, which 

 mighl produce bhe same result. Mo doubt we encounter occa- 

 sionally an erratic comet and are much puzzled by its great 

 luminosity and, in other respects, strange behaviour, as it 

 approaches our sun, but tho influence of a body of such small 

 mass upon our luminary is probably inappreciable. 



