A GIANT PLANET. 105 



This result is highly significant. If Jupiter showed 

 no belts and shone with a pure white colour, we could 

 explain it at once by simply regarding Jupiter as 

 wholly cloud-covered or snow-covered (for snow and 

 cloud shine with nearly equal lustre when similarly 

 illuminated). But the great dark belts which occupy 

 so large a proportion of the planet's disc altogether 

 negative this supposition. We seem compelled to be- 

 lieve that some considerable portion of the planet's 

 lustre is inherent. 



Let us, however, proceed carefully here. We have 

 to inquire first how far Zollner's results can be trusted, 

 and secondly, whether they are corroborated by any 

 independent evidence. Now Zollner carefully estimated 

 the weight of his observations, we may say he jealously 

 estimated their weight, for it must be remembered 

 that he was in no way interested in securing a greater 

 or less result, while he was greatly interested in so 

 stating the value of his results that those who might 

 succeed him in the inquiry should not detect any 

 serious error in his estimate. But his opinion of the 

 probable degree of error in his observations was such 

 as scarcely to affect to an appreciable extent the state- 

 ments we have made above. Taking Zollner's lowest 

 estimate of Jupiter's brightness, that statement remains 

 appreciably correct. 



And next as to corroborative evidence. 



It happens that we have a very delicate means of 

 measuring the degree of Jupiter's luminosity, as com- 

 pared with that of other orbs similarly placed. For 



