2 DeWitt B. Brace, 



since the brightness at any point depends on the depth of the 

 luminous layer and the solid angle which it subtends at that 

 point. 



The researches of both Herschel and Struve prove a non- 

 uniformity of distribution in all directions, but a concentration 

 of stars toward the medial plane of the Galaxy, more marked 

 the smaller the magnitude of the star. Struve, from conclu- 

 sions based on the supposition of an average uniformity of 

 stellar distribution in layers parallel to this central plane, and 

 on the assumption that the brightness is a measure of the 

 relative distance, attempted to prove that absorption must 

 take place. In fact, for a uniform distribution the number of 

 calculated stars of different magnitude should vary inversely 

 as their brightness. Now the number of calculated stars of 

 any magnitude exceeds slightly the number observed, this 

 excess being greater with the diminution of the magnitude. 

 Hence it is concluded that absorption must take place to 

 explain this increasing discrepancy, and that there must be 

 a limit to the space-penetrating power of a telescope jnuch 

 lower than the enfeeblement of light with the distance would 

 require. Later investigations regarding the constitution of 

 the visible universe show that Struve' s assumptions were 

 false, and that no law of uniformity in distribution or in 

 intrinsic brightness can be accepted. While it is at present 

 impossible to ascertain with much accuracy the real magni- 

 tudes of the stars, there are sufficient data to show that both 

 their volume and their intrinsic brightness, per unit surface, 

 vary between wide limits. The annual parallax of several 

 stars, as measured by different observers, gives approximately 

 consistent results. A comparison of the brightness and dis- 

 tances of these stars with the intrinsic brightness and dis- 

 tance of the sun, as made by Zollner, shows that the sun's 

 volume is but a small fraction of that of these stars, suppos- 

 ing equal intrinsic brightness per unit of surface. This 

 method of comparison, when applied to Sirius, gives a much 

 greater volume than other methods would warrant, though 

 far exceeding that of the sun in any case. It must hence 



2 



