378 SPECIAL RESULTS IN THE TJftANOLOGICAL 



point, the motion of the spots gives the true time of rota- 

 tion. These spots sometimes divide into two or more, also 

 resembling in this respect the spots of the Sun, as was 

 already recognised by Dominique Cassini in 1665. 



In the equatorial zone of Jupiter there are two broad prin- 

 cipal bands or belts of a grey or greyish-brown colour, which 

 become paler towards the edges, and gradually fade away 

 altogether. Their boundaries are very unequal and variable, 

 and they are separated from each other by an intermediate 

 quite bright equatorial streak. Towards both poles, also, 

 the entire surface is covered with many narrower, paler, 

 often interrupted, and even delicately-branched streaks or 

 bands, always parallel to the Equator. " These phseno- 

 mena," says Arago, "are most easily explained by the 

 hypothesis of an atmosphere partially obscured by strata of 

 clouds, but which, in zones adjacent to the Equator, is freed 

 from obscuring vapours, and rendered diaphanous probably 

 by the effect of trade- winds. Since (as was already assumed 

 by William Herschel, in a memoir which appeared in 1793, 

 in the 83d volume of the Philosophical Transactions) the 

 surface of clouds reflects a more intense light than the sur- 

 face of the planet itself, so the part of the surface which we 

 see through the clear air must appear darker than the cloudy 

 strata which reflect much light. Therefore, grey (or dark) and 

 bright bands alternate with each other : when the visual ray 

 from the eye of the observer is directed at small angles 

 obliquely towards the margin of the planet, the grey bands 

 are seen through a more considerable and thicker mass of 

 atmospheric strata, reflecting a greater quantity of light, 

 and appear less dark as they recede from the centre towards 

 the margin" ( 6l ?). 



