THE PLANET SATURN IN JULY 1869. 
259 
as the asteroids are exceedingly minute when compared with 
Jupiter, it is evident that those members of the system which 
had periods commensurable with his would be very largely 
disturbed, and so come to have another period. Thus we can 
understand the fact that there should be no asteroids at those 
particular mean distances from the sun which correspond to the 
particular periods in question. 
But it is clear that if there were any possibility of doubting 
the fact that the asteroids form a zone of disconnected bodies, 
the circumstance established by Professor Kirkwood would prove 
that fact. If, then, we can trace in the Saturnian ring- system 
any signs of the action of similar processes, we shall have an 
independent and perfect proof that the rings are not continuous, 
but composed of discrete satellites. Now this is precisely what 
Professor Kirkwood has been able to do. He has shown that a 
small satellite revolving in the space between the outer and 
inner rings — that is, travelling around the black division — 
would have a period commensurable not merely with that of the 
neighbouring Saturnian satellite, Enceladus, but with those of 
all the four inner satellites. It remains absolutel}^ certain, 
therefore, that the ring is composed of bodies moving freely in 
definite orbits. And, further, those who agree with me in 
accepting the nebular hypothesis (or a modification of it) as 
truly representing the mode in which the solar system reached 
its present condition, will see in the law established by Professor 
Kirkwood the action of one of the processes which must have 
been most effective in the formation of our system. 
This paper would be incomplete if I did not refer to the- 
information which Mr. Browning, F.K.A.S., the optician, has- 
recently obtained respecting the variety of colour observable in 
the Saturnian system. I had never been able to recognise any 
well-marked signs of colour on Saturn with a four-inch achro- 
matic refractor.* But not only has Mr. Browning himself been 
able to detect a variety of tints with his large reflector, but I 
have seen a letter from an observer (using a similar but smaller 
instrument) who refers to the same tints. These tints are thus 
referred by Mr. Browning to the well-known colours of the 
paint-box : — 
The rings yellow-ochre, shaded with the same and sepia. 
The globe yellow-ochre and brown madder, orange and purple, 
shaded with sepia. The crape-ring, purple madder and sepia. 
The great division in the rings, sepia. The pole and the 
narrow belts, situated near to it on the globe, pale cobalt blue. 
* It must be remembered that small apertures are more favourable, as a 
rule, for the exhibition of colour than large ones. In the case of Saturn, 
perhaps, this rule should rather be, large apeHures and high powers.’’ 
