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POPULAR SCIENCE REVIEW. 
perceive their inherent light in the central parts of the disc, which 
would therefore look brighter than the parts near the edge. This 
last is the phenomenon actually observed, but it does not of itself 
suffice to prove (though rendering it highly probable) that the 
light from the middle portion of the disc is in part inherent. 
Nevertheless the planet’s surface might, as I have already said, 
be intensely hot, and yet no trace of the inherent light be per- 
ceptible by us. That, however, could only happen because of 
the existence of very deep cloud-layers entirely shrouding the 
glowing planet, and in this case, as the clouds would probably 
— like our own clouds — have a much higher reflective capacity 
than rock surfaces have, we should expect to find the planets 
Jupiter and Saturn shining much more brightly, though only by 
reflected light, than they would if their surface resembled 
that of our own earth, or Mars, or Jupiter. Now the following 
table from Zollner’s 64 Grrundziige einer allgemeinen Photometrie 
des Himmels,” gives very interesting evidence on this point : — 
Snow just fallen reflects about 783 parts of 1000 of incident li, 
ght ; 
White paper 
tt 
a 700 „ 
a 
tt 
a 
Jupiter’s surface 
it 
„ 624 „ 
a 
a 
tt 
Saturn’s surface 
tt 
,, 498 ,, 
tt 
tt 
tt 
Uranus’s surface 
it 
„ 640 „ 
„ 
„ 
it 
Neptune’s surface 
it 
„ 465 „ 
tt 
tt 
a 
whereas 
White sandstone reflects only about 237 parts of 1000 of incident li< 
Clay marl 
it 
„ 156 
it 
it 
tt 
Mars’s surface 
it 
„ 267 
tt 
It 
a 
The Moon’s surface 
„ 174 
it 
tt 
tt 
We may take Jupiter and Saturn together, and Mars and 
the moon; getting average reflective power of giant planets 
: that of small planets:: 561 l 220 ; or the giant planets, if 
they owe their light entirely to reflection, have a reflective 
power more than 2J times greater than that of the small bodies, 
Mars and the moon. As the sea regions of Mars are observably 
darker than his land regions, it is probable that our earth, if her 
light could be estimated in the same way (by an observer on 
Mercury or Venus) would be found to have a smaller average 
reflective power than Mars, her seas being so much larger. 
We are forced by this argument to one of two conclusions — 
either Jupiter and Saturn shine in part by inherent light, or 
they are so thoroughly cloud-wrapped as to have a very high 
reflective power. Either conclusion would agree equally well 
with the theory I have advocated, though, of course, the former 
would be much more effective, and would in fact be quite de- 
cisive in its favour. 
