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no ocular evidence of this has ever been obtained, and of course the period is unknown. But the measures of Barnard showed a slight ellipticity of the disk which, if real, would prove a rapid rotation.

The spectroscope shows that the constitution of Uranus is materially different from that of any of the six planets which revolve between it and the sun. None of the latter gives a spectrum which is strikingly different from that of ordinary sunlight. But when the light of Uranus is spread out into a spectrum, a number of more or less shaded bands are seen, totally unlike the lines of an ordinary spectrum. Whether these bands are really what they appear, or whether they are composed of a multitude of fine dark lines invisible singly, owing to the faintness of the light, has not yet been ascertained; but the probabilities are that such is the case. Whether it is or not, the spectrum indicates that the light reflected from the planet has passed through a dense medium of a constitution quite different from that of our atmosphere. But it is as yet impossible to determine the nature of this medium.

There are four of these bodies moving round Uranus as he travels in his orbit. The two outer ones can be seen in a telescope of twelve inches aperture or more; the inner ones only in the most powerful telescopes of the world. The difficulty of seeing them does not arise from their small size, for they are probably nearly or quite as large as the others, but from their being blotted out by the glare of the planet.