194 LIGHT SCIENCE FOR LEISURE HOURS. 



of Jupiter has seemed to him irregular, as if the 

 planet's surface were partially flattened. Other obser- 

 vations tending in the same direction, as peculiarities 

 in the shape of the shadows of Jupiter's satellites on the 

 planet, in the shape of Saturn's shadows on his rings, and 

 so on, are of less weight perhaps than those already con- 

 sidered, but unless those who recorded them (including 

 some of the most skilful observers known) were entirely 

 deceived, such observations can only be fully explained 

 by the great depth of the cloud-laden atmosphere 

 which surrounds the giant planets. 



Lastly, there is the argument derivable from the 

 peculiar brightness of the planets Jupiter and Saturn. 

 These planets might be so hot as to glow with an intense 

 light and heat, yet no part of their light might be 

 discernible, the deep cloud-layers simply cutting it off 

 before it reached the outermost or visible cloud surface. 

 Or this might happen with all the rays except those 

 which travelled the shortest way through the cloud- 

 layers. In the former case we should perceive some of 

 the inherent light of these planets, in the latter we 

 should only perceive their inherent light in the central 

 parts of the disc, which would therefore look brighter 

 than the parts near the edge. This last is the phe- 

 nomenon actually observed, but it does not of itself 

 suffice to prove (though rendering it probable) that the 

 light from the middle portion of the disc is in part 

 inherent. Nevertheless the planet's surface might, as 

 I have already said, be intensely hot, and yet no trace 

 of the inherent light be perceptible by us. That, 



