22 The Saturnian System. (January, 
two or three hundred miles across would be quite invisible 
from the earth with the best telescopes and the most piercing 
eye-sight. In faé¢t, the apparent area of such openings in 
Jupiter’s atmosphere would be less than the fiftieth part of 
the least of Jupiter’s satellites. 
All that we might expect, I think, as an effect of the 
high temperature which our theory requires in the giant 
planets, is that these globes should shine with a light 
notably brighter than globes of equal size, similarly placed, 
and constituted of some such substance as the whiter kinds 
of sandstone. Now this is certainly the case. The lowest 
estimates of the brightness of Jupiter and Saturn assign to 
these orbs regarded as absolutely opaque a reflective power 
more than twice as great as that of white sandstone. 
Jupiter’s brightness, as a whole, is not far inferior to that 
which he would have if his whole surface were of the white- 
ness of driven snow. When we remember how far his 
globe is from being white, its actual whiteness as a whole 
resulting from the combination of several colours, we see 
that, according to this the lowest estimate, he must possess 
some inherent lustre. But other estimates have placed his 
brightness far higher. And it is a notable circumstance 
that Dr. De la Rue, in photographing Jupiter and our moon 
under the same circumstances (atmospheric and otherwise), 
found that the actinic power of the moon is to Jupiter’s 
only as 6 to 5, or 6 to 4, whereas if the two bodies both 
shone only by refleCting solar light, and possessed equal 
reflective powers, the moon should have nearly 27 times the 
actinic power of Jupiter, since Jupiter is 5; times further 
from the sun than the moon is. ‘‘ On December 7, 1857, 
Jupiter was photographed in 5 seconds, and Saturn in 60, 
and on another occasion the moon and Saturn were photo- 
graphed in 15 seconds, just after an occultation of the 
planet.” Jupiter should exceed Saturn about four times if 
both shone by reflecting solar light, and Saturn would 
appear, from the observation of December 7, 1857, to be of 
a brightness inferior to that due to its greater distance, 
while the other observation would imply the reverse. It 
seems manifest that photographic results would require 
a careful comparison, not only inter se but with some standard, 
to lead to satisfa¢tory conclusions. 
We may place greater reliance on direct photometric 
estimates; and, as it seems to me, the following results by 
Zollner, when carefully studied, indicate that the condition 
of the outer planets differs essentially from that of the earth, 
Mars, Moon, and probably Venus and Mercury. He found 
