296 THE POPULAR SCIENCE MONTHLY. 



latter, and more trustworthy series, Jupiter does not indeed shine quite 

 so brightly as Prof. Bond supposed, but the planet yet shines three 

 times as brightly as a globe of equal size would shine, if similarly 

 placed, but constituted like Mars, and four times as brightly as such 

 a globe would shine if constituted like our moon. Jupiter shines, in 

 fact, very nearly as brightly as though he were constituted like one of 

 our terrestrial clouds ! 



This result is highly significant. If Jupiter showed no belts and 

 shone with a pure white color, 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 disk altogether negative this supposition. We seem compelled 

 to believe that some considerable portion of the planet's lustre is in- 

 herent. 



Let us, however, proceed carefully here. We have to inquire first 

 how far Zullner'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 es- 

 timated 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 inter- 

 ested 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 aflect to an appreciable extent the statements 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 compared with that of other orbs 

 similarly placed. For his satellites pass across his face, and nothing 

 can be easier than to observe whether they appear darker or brighter 

 than his surface. 



It was an observation such as this which Mr. Lassell had made on 

 the night when he noticed the ruddiness of Jupiter's great medial belt. 

 By a singular chance Father Secchi made a similar observation during 

 his researches, and the reader will see, when we have quoted the nar- 

 ratives of both these observers, that the comparative darkness of all 

 four satellites will have been established. " The fourth satellite," says 

 Lassell, " has begun again for a season to cross the planet's disk, and 

 I have looked out for opportunities of observing its passages, and was 

 favored on the night of the 30th December last by witnessing a part 

 of its passage under circumstances more than usually propitious. On 

 its first entrance it was scarcely to be distinguished from the edge, not 

 appearing at all as the others do, as a round bright spot. As it ad- 

 vanced, it grew gradually manifestly darker than the surface of the 



