three
lines produced by ring, planet, and ring are in a straight line. Of
course the spectrum, which is practically a very faint copy of the solar
spectrum, shows the principal dark Fraunhofer lines, so that the reader
must imagine these for himself, parallel to the one we show in the
figure. But Saturn and the ring are not standing still, they are
rotating, the eastern part (at E) moving towards us, and the western
part (W) moving away from us.[27] At E the line will therefore be
shifted towards the violet end of the spectrum and at W towards the red,
and as the actual linear velocity is greater the further we get away
from the centre of Saturn (assuming ring and planet to rotate together),
the lines would be turned as in Fig. 67 (2), but the three would remain
in a straight line. If the ring consisted of two independent rings
separated by Cassini's division and rotating with different velocities,
the lines would be situated as in Fig. 67 (3), the lines due to the
inner ring being more deflected than those due to the outer ring, owing
to the greater velocity of the inner ring.

[Illustration: Fig. 67.--Prof. Keeler's Method of Measuring the Rotation
of Saturn's Ring.]

Finally, let us consider the case of the rings, consisting of
innumerable particles moving round the planet in accordance with
Kepler's third law. The actual velocities of these particles would be
per second:--

At outer edge of ring 10.69 miles.
At middle of ring 11.68 miles.
At inner edge of ring 13.01 miles.
Rotation speed at surface of planet 6.38 miles.

The shifting of the lines of the spectrum should be in accordance with
these velocities, and it is easy to see that the lines ought to lie as
in the fourth figure. When Professor Keeler came to examine the
photographed spectra, he found the lines of the three spectra tilted
precisely in this manner, showing that the outer edge of the ring was
travelling round the planet with a smaller linear velocity than the
inner one, as it ought to do if the sources of light (or, rather, the
reflectors of sunlight) were independent particles free to move
according to Kepler's third law, and as it ought not to do if the ring,
or rings, were rigid, in which case the outer edge would have the
greatest linear speed, as it had to travel through the greatest
distance. Here, at last, was the proof of the meteoritic composition of
Saturn's r