THE PLANET SATURN IN JULY 1S69. 
255 
flat the total amount of light received from them (the ball being 
supposed removed) would be increased, through two causes, as 
the rings opened. Firstly, the increased apparent size of the 
luminous surface would have an obvious effect. Owing to this 
cause the illumination would vary as the sine of the angle at 
which the line of light from the earth is inclined to the plane 
of the rings. Secondly, the apparent brilliancy of each point of 
the ring-system would be increased as the sine of the angle at 
which the sun’s rays are inclined to the plane of the rings. 
Thus the total amount of light would increase as the product 
of these two sines, or assuming what is commonly the case, that 
the earth and sun are almost equally raised above the surface 
of the rings, the total amount of light received from the rings 
would vary as the square of either sine. 
But if the rings consist of a multitude of discrete satellites, 
there must result a different state of things. Take a single 
satellite, and we see at once that so long as the whole of this 
satellite can be seen we get the same amount of light from it, 
whatever the elevation of the sun above the mean plane of the 
rings. And though the problem seems to get somewhat com- 
plicated when we consider the case of a multitude of satellites, 
yet it will be found, on examination, that there is no longer the 
same variation to be looked for as was shown to exist in the 
former case, owing to the sun’s change of elevation. In fact, 
we have a case somewhat resembling that of the moon ; the 
illumination of whose disc has been shown by Zollner not to 
diminish towards the edges according to the varying inclination 
of the solar rays to the moon’s surface, but rather to increase ; 
while calculation has shown the probable reason to consist in 
the fact that the moon is not a smooth globe, but covered with 
hills and mountains, whose sides are inclined at greater or less 
elevations to the mean level of the lunar surface. 
This being so, two means of observation seem available. 
First, a definite part of the ring’s width might be compared 
with the equatorial bright belt of the planet ; the brilliancy of 
that belt being we may assume constant. This method would 
probably involve difficulties ; but from the success with which 
Mr. Browning gauged the relative brilliancy of different parts 
of the disc of Jupiter last spring, I have no reason to doubt that, 
with suitably prepared and graduated darkening glasses, the 
comparison might be satisfactorily carried out. Then the 
change of brilliancy of the particular part of the ring examined, 
as the system gradually closed, would afford evidence of the 
nature of that portion of the ring, according to the principles 
enunciated above. Secondly, a process might be applied to 
Saturn corresponding to that which Dr. Zollner recently applied 
to the planet Mars. By determining the total amount of light 
VOL. VIII. — NO. XXXII. S 
