THE SATELLITES OF JUPITER. 515 



probably in consequence of the trade-winds. Since, as 

 William Herschel already assumed in a treatise in the 

 83rd vol. of the Philosophical Transactions, which appeared 

 in 1793, the cloud-surface reflects a more intense light than 

 the surface of the planet, so that part of the ground which we 

 see through the clearer air must have less light (appear 

 darker) than the strata of clouds reflecting large quantities 

 of light. On that account grey (dark) and clear bands 

 alternate with each other ; the former appear so much the 

 less dark-coloured in proportion to the distance from the 

 centre, when, the visual radius of the observer being directed 

 obliquely towards the edge of the planet, at a small angle, 

 they are seen through a larger and thicker mass of atmosphere, 

 reflecting more light. 



THE SATELLITES OF JUPITEB. 



Even so early as the brilliant epoch of Galileo the correct 

 opinion was formed, that the subordinate planetary system 

 of Jupiter might present, in many relations of space and 



body of the planet is added the light reflected by the denser 

 zone. The bands would be, from this reason, less obscure the 

 greater the distance from the centre. Finally, at the very 

 edges of the planet's disc the light reflected by the zone seen, 

 in its greatest thickness, would cause the difference of inten- 

 sity which existed between the quantities of light reflected 

 by the planet and by the atmosphere of clouds, to disappear, 

 and the bands which exist only in virtue of that difference 

 would cease to be visible. Something analogous is observed 

 in mountainous countries ; in the neighbourhood of a forest 

 of fir-trees they appear black, but in proportion as the 

 observer removes to a greater distance, the interposed atmo- 

 spheric strata become thicker and thicker, and reflect light. 

 The difference of tint between the forest and the objects 

 near, diminishes more and more, and ends by their being 

 confounded together on removing to a sufficient distance." 



YOL. IV. Q 



