=====+================================================= | Diameter List +-----+-----+------+------+------+-------+-------- | 4″ | 6″ | 15″ | 25″ | 60″ | 200″ | >200″ -----+-----+-----+------+------+------+-------+-------- 3 | 205 | 322 | 291 | 280 | 195 | 36 | 6 4 | | 1 | 69 | 153 | 19 | 6 | 1 5 | | 6 | 99 | 106 | 23 | 1 | 3 6 | | 14 | 114 | 72 | 2 | 2 | 1 7 | | 9 | 103 | 156 | 35 | 4 | 12 8 | | 80 | 372 | 243 | 34 | 3 | 9 | | 48 | 174 | 160 | 19 | | 10 | | 3 | 31 | 26 | 2 | | 11 | | | 13 | 60 | 19 | 1 | 12 | | 14 | 61 | 162 | 27 | 10 | 13 | | | 20 | 61 | 26 | 3 | 14 | | 3 | 160 | 296 | 43 | 1 | 1 -----+-----+-----+------+------+------+-------+-------- The evidence, while far from conclusive, appears to indicate the existence of actual clusters of these small nebulae in the sky. If this is true, it is natural to suppose them physically connected, as is the case in star-clusters. It is not possible to form a conception of this state of affairs until some idea of their distance is acquired. Suppose them to be extra-sidereal and perhaps we see clusters of galaxies; suppose them within our system, their nature becomes a mystery. [Illustration: FIG. 2.--Distribution of size of nebulae in Wolf’s Perseus and three Yerkes fields] The question of nebular distances is of first importance, for it is in terms of this quantity that the various dimensions may be expressed. The dark nebulosities, by their very nature, and the great diffuse clouds, some obviously connected with even naked-eye stars, may safely be considered as galactic, and this view is in accord with their low radial velocities with reference to our system. The planetaries have repeatedly been measured for proper motion, with negligible results. Taking a value of 40 km/sec. for the average radial velocity, and an assumed lower limit of 0.02″ for the average annual proper motion, a tentative lower limit for the average distance of the largest, and hence probably the nearest, is found to be about 2000 light-years. There is thus no reason on this ground for placing them outside our system, especially in view of their decidedly systematic galactic distribution.