120 



Dr. Frank Watson Dyson 



[April 24, 



the formulation of dynamical laws on the constitution of the stellar 

 universe. 



5. I have chosen a small area round the pole as a sample, because 

 this part of the sky has been observed more fully than any other of 

 equal extent. It forms a small cap extending to a distance of D" 

 from the pole, and covering about y|o of the whole sky. In the 

 years 18o5-6 Carrington, an English amateur astronomer, well 

 known from his observations of sun-spots, using a very small transit 

 instrument, observed the positions of all the stars in tins part of 

 the sky from the brightest down to very faint stars between the 

 tenth and eleventh magnitudes. [An eleventh magnitude star is 

 exactly 100 times as faint as one of the sixth magnitude, and a sixth 

 magnitude stiir is the faintest star which can be seen with the naked 

 eye.] He thus constructed a catalogue, giving with great accuracy 

 the positions of 3700 stars for the year 1855. About the year 1900 

 these stars were re-observed at Greenwich by a combination of visual 

 and photographic observations. By comparison with the positions 

 as given in Carrington's catalogue, the angular movement of each 

 of these 3700 stars in 45 years is determined. These angular move- 

 ments, or proper motions as they are technically called, are the data 

 available for obtaining the actual positions and movements of the 

 stars in space. ^Ye have to solve the geometrical problem of 

 making these stars stand out in three dimensions, so that we may 

 see them as we see a picture in a stereoscope. 



6. Now the proper motions of stars are very small. The star of 

 largest proper motion moves only 870" a century. An idea of the 

 smallness of this motion may be obtained from the fact that it would 

 take two centuries to move a distance equal to the apparent diameter 

 of the sun or moon. It would take four centuries to move across 

 the small cluster of the Pleiades. There is no star among those 

 near the North Pole with a proper motion so great as one-tenth of 

 this. The following table gives an abstract of the proper motions 

 of the 3700 stars under consideration : — 



Table I. 



It is clear that the stars with large proper motions must either 

 be moving fast or must be comparatively near. These are the 



