January 23, 1903.] 



SCIENCE. 



123 



dominant and throughout its whole circuit 

 in other directions, a different class. 



This supposition was still farther em- 

 phasized through the researches of See- 

 liger on the distribution of the stars in 

 space. He exclaimed, with what we might 

 regard as a pardonable approach to en- 

 thusiasm not common in a mathematical 

 discussion, that the Milky Way was now 

 to be regarded as a single object. Another 

 curious fact is that, within it, the stars, 

 so far as we can yet determine, seem to be 

 equally scattered from one extreme to the 

 other. In two opposite directions, that of 

 the poles of the Milky Way, the number 

 of stars which we see are fewest. Their 

 thickness increases, slowly at first, then 

 more rapidly, until we reach the Milky 

 Way itself. So far as has yet been de- 

 termined there is a perfect symmetry on 

 the two sides of the Milky Way. If, on 

 one side the stars seem to be a little thicker 

 here than on the corresponding side, the 

 ease is the reverse in some other regions. 

 The general rule is that if we take two 

 diametrically opposite directions in the 

 heavens, no matter which, and count the 

 number of stars within a given area of, 

 say, ten square degrees in each of these 

 opposite directions, we shall find the num- 

 ber to be nearly the same. The nearer our 

 directions come to the plane of the Milky 

 Way, the more numerous the stars we shall 

 find in the two opposite cases, but the in- 

 crease in thickness will not be much 

 greater at one end of our line of sight 

 than at the opposite end. Moreover, if 

 we change the direction of this imaginary 

 diameter of the universe, we shall find 

 that, so long as it makes the same angle 

 with the Milky Way, so long will the num- 

 ber of stars around it remain the same. 

 The statistical evidence also shows us that 

 the stars of the Milky Way are, in a gen- 

 eral average, several times as bright as 

 those situated elsewhere. 



The feature of the universe which should 

 therefore command our attention is the ar- 

 rangement of a large part of the stars 

 which compose it in a ring, seemingly alike 

 in all its parts, so far as general features 

 are concerned. So far as research has yet 

 gone, we are not able to say decisively that 

 one region of this ring differs essentially 

 from another. It may, therefore, be re- 

 garded as forming a structure built on a 

 uniform plan throughout. 



All scientific conclusions drawn from 

 statistical data require a critical investiga- 

 tion of the basis on which they rest. If 

 we are going, from merely counting the 

 stars, observing their magnitudes and de- 

 termining their proper motions, to draw 

 conclusions as to the structure of the uni- 

 verse in space, the question may arise how 

 we can form any estimate whatever of the 

 possible distance of the stars, a conclusion 

 as to which must be the very first step we 

 take. We can hardly say that the par- 

 allaxes of more than 100 stars have been 

 measured with any approach to certainty. 

 The individuals of this 100 are situated 

 at very different distances from us. We 

 hope, by long and repeated observations, 

 to make a fairly approximate determina- 

 tion of the parallaxes of all the stars 

 whose distance is less than 20 times that 

 of a Centauri. But how can we know 

 anything about the distance of stars out- 

 side this sphere? What can we say 

 against the view of Kepler that the space 

 around our sun is very much thinner in 

 stars than it is at a greater distance; in 

 fact that the great mass of the stars may 

 be situated between the surfaces of two 

 concentrated spheres not very different in 

 radius. May not this universe of stars be 

 somewhat in the nature of a hollow sphere ? 



This objection requires very careful con- 

 sideration on the part of all who draw 

 conclusions as to the distribution of stars 

 in space and as to the extent of the visible 



