Slipher has recently measured the rotational speed of one "spindle" nebula, believed to be a spiral. He finds it to be enormously rapid; no motions in the solar system approach it in magnitude. The evidence is to the effect that the spirals are in general very far away;[3] perhaps on or beyond the confines of our stellar system, but not certainly so. Accordingly, we are led to believe that the spirals studied thus far have diameters 20 times or 100 times, or in some cases several thousand times, the diameter of our solar system. It is difficult to avoid the conclusion that in general they are immensely more massive than is our solar system. The spiral which has been assumed as the forerunner of our system must have been of diminutive size as compared with the larger and brighter spirals which we see to-day. [2] It would seem that all rotating nebulae should in reality possess some of the attributes of spiral motion. Whether the spiral structure should be visible or invisible to a terrestrial observer would depend upon the sizes and distances of the nebulae, upon the distribution of materials composing them, and perhaps upon other factors. See developed the hypothesis that spiral nebulae owe their origin to the collision of two nebulae. Collisions of this kind could readily occur because of the enormous dimensions of the nebulae, and motions of rotation and consequently spiral structure might readily result therefrom. The abnormally high speeds of the spiral nebulae are apparently a very strong objection to the hypothesis. [3] Bohlin found a parallax of 0"17 for the Andromeda Nebula, and Lampland thinks that Nebula N.G.G. 4594 has a proper motion of approximately 0"05 per annum. We are sadly in need of information concerning the constitution of the spiral nebulae. Their spectra appear to be prevailingly of the solar type, except that a very small proportion contain some bright lines in addition to the continuous spectrum. So far as their spectra are concerned, they may be great clusters of stars, or they may consist each of a central star sending its light out upon surrounding dark materials and thus rendering these materials visible to us. The first alternative is unsatisfactory, for all parts of spirals have hazy borders, as if the structure is nebulous or consists of irregular groups of small masses; and the second alternative is unsatisfactory, for in many spirals the most outlying masses seem to be as bright as masses of the same areas situated only one half as far from the center, whereas in general the inner area should be at least four times as bright as the outer area. All astronomers are ready to confess that we do not know much about the conditions existing in spiral nebulae. THE EARTH-MOON SYSTEM