In the sun and in Jupiter alike we have a view, not of the true surface, but of an envelope which seems to represent the point of condensation of currents of matter thrown up from depths below--an envelope agitated in both cases, though more slowly in that of Jupiter, by disturbances which bear witness to the operation of stupendous forces beneath its veil. In both bodies there is a similar small density: neither the sun nor Jupiter is much denser than water; in both the determination of the rotation period is complicated by the fact that the markings of the bright envelope by which the determinations must be made move with entirely different speeds in different latitudes. Here, however, there is a divergence, for while in the case of the sun the period increases uniformly from the equator to the poles, there is no such uniformity in the case of Jupiter. Thus certain dark spots in 25° north latitude were found in 1880 to have a shorter period than even the swift equatorial white markings. One further circumstance remains to be noted in pursuance of these resemblances. Not only does the disc of Jupiter shade away at its edges in a manner somewhat similar to that of the sun, being much more brilliant in the centre than at the limb, but his remarkable brilliancy, already noticed, has given rise to the suggestion that to some small extent he may shine by his own inherent light. There are certain difficulties, however, in the way of such a suggestion. The satellites, for example, disappear absolutely when they enter the shadow of their great primary--a fact which is conclusive against the latter being self-luminous to anything more than a very small extent, as even a small emission of native light from Jupiter would suffice to render them visible. But even supposing that the idea of self-luminosity has to be abandoned, everything points to the fact that in Jupiter we have a body which presents much stronger analogies to the sun than to those planets of the solar system which we have so far considered. The late Mr. R. A. Proctor's conclusion probably represents the true state of the case with regard to the giant planet: 'It may be regarded as practically proved that Jupiter's condition rather resembles that of a small sun which has nearly reached the dark stage than that of a world which is within a measurable time-interval from the stage of orb-life through which our own Earth is passing.'