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operation without appreciably affecting the apparent width of 
the ring-system. And certainly the present appearance of the 
dark ring is such as to encourage the View that sufficiently rapid 
changes are in progress. 
It would follow from these views, that the spectrum of the 
ring’s light would exhibit variations corresponding to the various 
parts of the ring’s breadth. Of course, there are already well- 
marked gradations of light in the spectrum, because the light is 
different in different parts of the ring’s breadth. But the dark 
lines I have already spoken of as distinctive of the ring’s spec- 
trum, ought to be more distinctly seen in certain parts of the 
ring on another account. For there can be little doubt that 
the central parts of each ring are those at which collisions take 
place most frequently between the satellites ; and, therefore, if 
the cause I have been considering is really in operation, the 
dark lines ought to be seen best in those parts of the spectrum’s 
width which correspond to the central portions of the rings. 
The observation might be worth making, though it would be 
one of great difficulty and delicacy. 
Some recent researches by Professor Kirkwood, of Illinois, 
have supplied an interesting and sound proof of the real struc- 
ture of the rings. They are particularly interesting to myself, 
as affording an unexpected proof of a view I had put forward 
some time since which had seemed to some to be more imaginative 
than well-founded. In the preface to my treatise on Saturn, 
I had said that possibly we may yet detect in the Saturnian 
rings the indications of those processes by which the solar system 
had reached its present state. Now Professor Kirkwood’s re- 
searches tend directly to establish such a relation. 
He had shown that when the asteroids are arranged in the 
order of their mean distances certain well-marked gaps are 
observable, and that these gaps correspond to those mean 
distances which would give periods commensurable with the 
period of Jupiter. We know that when a planet has a period very 
nearly commensurable (according to some simple relation) with 
the period of a neighbouring planet, the two bodies disturb each 
other much more effectively than they would if there were no 
such relation. If one of the planets be much larger than the 
other, far the larger part of the disturbance falls upon the 
motions of the smaller planet. Saturn, for example, had long 
since been noticed as having his motions affected by a very 
remarkable inequality ; and the search for a cause resulted in 
the discovery that the peculiarity is due to the relation existing 
between the motions of Saturn and Jupiter, by which two revo- 
lutions of tlie former planet are accomplished in about the same 
time as five of the latter. The disturbance falls principally on 
Saturn, as being so much the smaller of the two bodies. And, 
