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of 3000 stars which he determined, only a few are available 

 for the purpose. Even since his time, the determinations 

 made by each generation of astronomers have not been suf- 

 ficiently complete and systematic to furnish the material for 

 anything like a precise determination of the proper motions 

 of stars. To determine a single position of any one star 

 involves a good deal of computation, and if we reflect that, 

 in order to attack the problem in question in a satisfactory 

 way, we should have observations of 1,000,000 of these 

 bodies made at intervals of at least a considerable fraction 

 of a century, we see what an enormous task the astronomers 

 dealing with this problem have before them, and how 

 imperfect must be any determination of the distance 

 of the stars based on our motion through space. So 

 far as an estimate can be made, it seems to agree 

 fairly well with the results obtained by the other methods. 

 Speaking roughly, we have reason, from the data so far 

 available, to believe that the stars of the Milky Way are 

 situated at a distance between 100,000,000 and 200,000,000 

 times the distance of the sun. At distances less than this it 

 seems likely that the stars are distributed through space with 

 some approach to uniformity. We may state as a general 

 conclusion, indicated by several methods of making the 

 estimate, that nearly all the stars which we can see with our 

 telescopes are contained within a sphere not likely to be much 

 more than 200,000,000 times the distance of the sun. 



The inquiring reader may here ask another question. 

 Granting that all the stars we can see are contained within 

 this limit, may there not be any number of stars outside 

 the limit which are invisible only because they are too far 

 away to be seen? 



This question may be answered quite definitely if we 

 grant that light from the most distant stars meets with no 

 obstruction in reaching us. The most conclusive answer 

 is afforded by the measure of starlight. If the stars ex- 

 tended out indefinitely, then the number of those of each 

 order of magnitude would be nearly four times that of the 

 magnitude next brighter. For example, we should have 

 nearly four times as many stars of the sixth magnitude as 

 of the fifth; nearly four times as many of the seventh 



