THE EXTENT OF THE UNIVERSE 333 



The probabilities are in favor of the result giving the 

 greater thickness of the stars. But, after all, the discrep- 

 ancy does not change the general conclusion as to the limits 

 of the visible universe. If we cannot estimate its extent 

 with the same certahity that we can determine the size of 

 the earth, we can still form a general idea of it. 



The estimates we have made are based on the supposition 

 that the stars are equally scattered in space. We have good 

 reason to believe that this is true of all the stars except 

 those of the Milky Way. But, after all, the latter prob- 

 ably includes half the whole number of stars visible with 

 a telescope, and the question may arise whether our results 

 are seriously wrong from this cause. This question can 

 best be solved by yet another method of estimating the 

 average distance of certain classes of stars. 



The parallaxes of which we have heretofore spoken con^ 

 sist in the change in the direction of a star produced by the 

 swing of the earth from one side of its orbit to the other. 

 But we have already remarked that our solar system, with 

 the earth as one of its bodies, has been journeying straight- 

 forward through space during all historic times. It follows, 

 therefore, that we are continually changing the position 

 from which we view the stars, and that, if the latter were 

 at rest, we could, by measuring the apparent speed with 

 which they are moving in the opposite direction from that 

 of the earth, determine their distance. But since every star 

 has its own motion, it is impossible, in any one case, to 

 determine how much of the apparent motion is due to the 

 star itself, and how much to the motion of the solar system 

 through space. Yet, by taking general averages among 

 groups of stars, most of which are probably near each 

 other, it is possible to estimate the average distance by 

 this method. When an attempt is made to apply it, so as 

 to obtain a definite result, the astronomer finds that the 

 data now available for the purpose are very deficient. The 

 proper motion of a star can be determined only by comparing 

 its observed position in the heavens at two widely separate 

 epochs. Observations of sufficient precision for this purpose 

 were commenced about 1750 at the Greenwich Observatory, 

 by Bradley, then Astronomer Royal of England. But out 



