1874.] The Saturman System. aI 
disc does not show that the light is im the main reflected, 
though it proves that a considerable share of the planet’s 
light is reflected. The apparent blackness of the satellites’ 
shadows would be a strong argument if we were not aware 
how large a part contrast may play in producing such a 
phenomenon. The spots on the sun look black at their 
nucleus by contrast with the light of the photosphere, but 
so does the glowing lime of the oxyhydrogen light look 
black against the sun’s disc. And a like test is available to 
show that the blackness of the shadows of Jupiter’s satel- 
lites may be apparent only. For the satellites themselves 
ordinarily look dark in transit, and the fourth satellite looks 
absolutely black. Now this fourth satellite is certainly not 
black ; in fact, it shines on the background of the sky with 
a sufficiently bright light. We have, therefore, no evidence 
that the inherent luminosity of Jupiter may not be equiva- 
lent to the light which the fourth satellite reflects. 
Now it is to be noticed that the light of matter glowing 
with intensity of heat is not necessarily intense. The red 
of burning coals, for example, or of hot metal is not so 
bright as many imagine. It is not so bright as a red object 
in ordinary sunlight. The very best imitation of a fire of 
glowing coals I ever remember to have seen was produced 
by a piece of rather dark red cloth, accidentally placed ina 
grate in an ill-lighted room (day-light). Bright red cloth in 
sunlight does not produce nearly so good an illusion; and 
it is perhaps hardly necessary to remark that the ordinary 
imitations of a coal-fire in theatrical representations fail in 
consequence of their being very much too bright. 
In my opinion, there is nothing to prevent us from 
assuming that the darker belts of Jupiter owe the ruddiness 
of their colour to the glow of a red-hot interior, and this 
extends to the case of Saturn, whose dark belts bear 
a tolerably close resemblance to those of Jupiter in their 
general tint. I would not have it thought, however, that I 
limit the inherent heat of Jupiter to ordinary red heat. 
I simply infer that the portion of the inherent light which 
makes its way through the cloud-laden envelope of the 
dark belts corresponds to that of red-hot matter. I should 
regard the brighter belts, notwithstanding their greater 
brightness, as those whence less inherent luminosity 
proceeds, the greater part of their light being, I conceive, 
reflected from clouds of purest white, concealing the glowing 
surface beneath. It is to be carefully noted, however, that 
we have no reason for believing that any part of the cloud- 
belts of either planet is absolutely continuous. Openings 
