XVI.] THE DISTRIBUTION OF CHEMICAL GROUPS OF STARS. 139 



grasped by considering what happens to any one, travelling in a 

 railway train. If the train be going fairly quickly, and we look 

 at the near objects, we find that they appear to rush by so rapidly that 

 they tire the eye ; the more distant the object we look at is the more 

 slowly it appears to move, and the less the eye is fatigued. Now, sup- 

 pose that instead of the train rushing through the country and passing 

 the objects which we regard under these different conditions, the dif- 

 ferent objects are rushing past us at rest. Then, obviously, those 

 things which appear to be moving most quickly will be those nearest, 

 and the more distant objects, just because they are distant, will appear 

 to move more slowly ; that is to say, we shall get what is called a large 

 " proper motion " in the case of the objects nearest to us, and a small 

 " proper motion " in the case of the bodies which are further away. 



This question has been attacked with regard to the stars in mag- 

 nificent fashion by a great number of astronomers. 



It was Mr. Monck who was the first to show in 1892* that the 

 gaseous stars had the smallest proper motion ; that is to say, that the 

 hottest stars were further away from us than the cooler ones. He 

 next found that the proto-metallic stars that is to say, the stars not 

 so hot as the gaseous, but hotter than the metallic stars had the next 

 smaller proper motion. This, of course, indicates that the metallic 

 stars are the nearest to us unless proper motion does not depend upon 

 distance, but rather upon a greater average velocity in space. It has 

 been shown, however, by considering the sun's movement in space, that 

 this view probably may be neglected. The first discussion of proper 

 motion, then, went to show, roughly, that the hotter a star is the 

 further away from us it is ; and it made out a fair case for the conclu- 

 sion that the sun forms one of a group or cluster of stars in which the 

 predominating type of spectrum is similar to its own. 



Kapteyn carried the inquiry a stage further.! Working upon the 

 idea that stars with the greatest proper motion are on the average the 

 nearest, the part of the piroper motion due to the sun's translation in 

 space he considered must depend strictly upon the distance, and he 

 determined this by resolving the observed proper motion along a great 

 circle passing through the point of space towards which the sun is 

 moving, which is called the apex of the sun's way, and reducing to a 

 point 90 from the apex. His results were practically the same as 

 those obtained by taking the individual proper motions. He also 

 found that stars with the greatest proper motion are mainly metallic, 

 and have no regard at all to the Milky Way ; that stars with the 

 smallest and no observable proper motion are gaseous and proto- 



* Astronomy and Astro-Physics, vol. xviii, 2, p. 876. 

 f Amsterdam Academy of Science, 1893. 



