This motion, whether it is towards or away from the central, lens-shaped portion, would cause a double streaming motion in that central portion of the kind we have found in our own system. Again, and altogether apart from these considerations, there are good reasons for supposing our Milky Way to possess a double-armed spiral structure. And the great patches of dark absorbing matter which are known to exist in the Milky Way (see Fig. 22) would give very much the mottled appearance we notice in the arms (which we see edge-on) of the nebula in Coma Berenices. The hypothesis, therefore, that our universe is a spiral nebula has much to be said for it. If it be accepted it greatly increases our estimate of the size of the material universe. For our central, lens-shaped system is calculated to extend towards the Milky Way for more than twenty thousand times a million million miles, and about a third of this distance towards what we have called the poles. If, as we suppose, each spiral nebula is an independent stellar universe comparable in size with our own, then, since there are hundreds of thousands of spiral nebulae, we see that the size of the whole material universe is indeed beyond our comprehension. [Illustration: _Photo: Mount Wilson Observatory._ FIG. 26.--A SPIRAL NEBULA SEEN EDGE-ON Notice the lens-shaped formation of the nucleus and the arm stretching as a band across it. See reference in the text to the resemblance between this and our stellar universe.] [Illustration: _Photo: H. J. Shepstone._ 100-INCH TELESCOPE, MOUNT WILSON A reflecting telescope: the largest in the world. The mirror is situated at the base of the telescope.] [Illustration: