Fig. 4. Internal motions in Messier 101. The arrows indicate the direction and magnitude of the mean annual motions. Their scale (0”1) is indicated on the plate. The scale of the nebula is 1mm = 10”5. The comparison stars are enclosed in circles. From _Proceedings of the National Academy of Sciences of U. S. A._] [Illustration: Fig. 5. Spiral nebula No. 51 in Messier’s Catalogue; situated in Canes Venatici, and photographed February 7 and 8, 1910, from Mount Wilson Observatory in California. Scale: 1 millimeter = 5 sec. arc. Lyran Herkules Ormbäraren Örnen Skölden Svanen Räven Pilen Ormbäraren Skytten Skorpionen Södra Kronan Kolsäck Syd-Korset Skeppet Argo Kölen Skeppet Argo Styre Segel N Enhörningen Enhörningen Kusken Cepheus Perseus Kassiopeja Svanen ] It is clear that the Milky Way may have been formed through the collision of two immense nebulous gas-aggregations in the manner just described. Subsequently and by virtue of the magnitude of the Galaxy great quantities of wandering cosmic matter and minor stellar bodies have been gathered in occasionally accompanied by larger clusterings such as the planetary nebulæ referred to. How well justified we are in looking upon the Milky Way as a spiral nebula is apparent from the picture (Fig. 5) reproducing a photograph of the familiar regular nebula in the Dogs (Canes Venatici). It shows a wealth of detail hitherto not surmised. The feat was accomplished in the Carnegie Observatory on Mount Wilson in Southern California with the help of optical resources vastly superior to all earlier means. The Milky Way has previously been compared to this nebula, but due to deficient enlargement their striking similarity has not been fully recognized until now.