Such a discous nebula has been investigated by the astronomer F. G. Pease of the Carnegie Observatory on Mount Wilson. By means of the spectroscope he has studied the motion of Nebula No. 4594 in the New General Catalogue (see Fig. 3). This body is believed to be a spiral nebula like those in Figs. 4 and 5 but viewed from the side so that the spirals appear as a band. As the picture shows, this band is coursed through by a thick dark line, owing, it is believed, to a cold non-luminous dust-cloud outside of the spiral. The bright band is broadest in the centre. The curvature of the dark middle line in the shape of an arc, whose apex points downward, combined with the fact that the major portion of light falls above this arc, indicates that we do not view the nebula exactly on edge but from the upper (north) side of a plane through the arms of the nebula. The nucleus speeds away from us at the dizzying rate of 1180 km. (730 miles) per second. The east edge, _i. e._ the left on the picture, departs with the still higher velocity of 1630 km. (1100 miles) per second, while the west (right) edge retreats at the rate of only 800 km. (495 miles) per second. According to Pease, the nebula rotates as a solid disk so that the difference between the velocity of any point and that of the centre increases in the same proportion as the distance of the point in question from the centre. It is probable that we have been prevented from observing the outer parts, corresponding to the spiral arms proper, by the ring of dust which encircles the nebula. The visible portion occupies an arc of 2¼ minutes on each side of the centre. Its spectrum corresponds to that of star-group F-5 among the yellow stars in the Harvard classification. Therefore, it is not the light of the coherent gas-cloud which preponderates but rather that of the stars consolidated within the cloud, and corresponding to the stars in the Milky Way. This star-light is so bright that it entirely suppresses the radiation from the gas-cloud itself.