196 LIGHT SCIENCE FOR LEISURE HOURS. 



(by an observer on Mercury or Venus) would be found 

 to have a smaller average reflective power than Mars, 

 her seas being so much larger. 



We are forced by this argument to one of two con- 

 clusions either Jupiter and Saturn shine in part by 

 inherent light, or they are so thoroughly cloud-wrapped 

 as to have a very high reflective power. Either con- 

 clusion would agree equally well with the theory I have 

 advocated, though, of course, the former would be 

 much more effective, and would in fact be quite decisive 

 in its favour. 



For my own part, I think that the photometric 

 evidence renders it very probable that a slight portion 

 of the light of the planets Jupiter and Saturn is 

 inherent ; and I think the colour of the equatorial belt 

 of Jupiter and its changes of colour correspond with 

 this view. I should be disposed to assign, as the 

 reflective power of Jupiter (his albedo, as Zollner calls 

 it), about 500, or more than twice the reflective power 

 of white sandstone, and thus to attribute about one- 

 fifth of Jupiter's light to the planet's inherent lustre. 

 (In Saturn's case /ollner's observations are much less 

 satisfactory his measures indeed of the planet's total 

 light were probably even more satisfactory than in 

 Jupiter's case, but it is exceedingly difficult to take 

 properly into account the effect of the ring-system, 

 which, though very much foreshortened when Zollner 

 made his observations, must nevertheless have ap- 

 preciably affected his results.) All the known facts 

 accord well with this view. 



