302 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1908. 



Berenice," we shall clearly perceive this gradual and regular change 

 in the number of stars. Now, if we repeat the same process, be- 

 ginning from some other point of the Milky Way, say in Cassiopeia, 

 or the Southern Cross, we shall find that, not only is there a similar 

 gradual change, but we shall approximately go through the same 

 changes. 



LITTLE VARIATION WITH GALACTIC LONGITUDE. 



At the same distance from the Milky Way we shall find approxi- 

 mately the same number of stars in the field of the telescope. 



Put in other words: The richness of stars varies regularly with 

 the galactic latitude; it varies relatively little with the galactic 

 longitude. 



Imitating most of the investigators of the stellar system, we will 

 therefore disregard the longitude and keep in view only the changes 

 with the galactic latitude. In reality this comes to being satisfied 

 with a first approximation. For, in reality, there are differences in 

 the different longitudes, especially in the Milky Way itself. But 

 even here the differences are not so great as seems commonly to be 

 supposed. There is every reason to believe, therefore, that our ap- 

 proximation will be already a tolerably close one. 



REAL STRUCTURE. 



Meanwhile what the Herschel gauges teach us is only relative to 

 the outward appearance of the sky. What is the real structure of 

 the stellar world? If we see so many stars in the field, with the 

 telescope directed to the Milky Way, is it because they are really more 

 closely crowded there, as Struve thinks, or is the view of the older 

 Herschel correct, who imagined that the greater richness is simply a 

 consequence of the fact that we are looking in deeper layers of stars; 

 that our universe is more extensive in the Milky Way than it is in 

 other directions ? 



Imagine that we could actually travel through space. For in- 

 stance, imagine that first we travel in the direction of the constella- 

 tion Cassiopeia. If we travel with the velocity of light, not so very 

 many years would pass before we get near to some star. Proceeding 

 on our journey for many, many more years, always straight on, we 

 will pass more stars by and by. How will these stars look thus 

 viewed from a moderate distance — say, from a distance as that of 

 the sun? 



Will they all be found to be of equal luminosity, as Struve prac- 

 tically assumed? And in this case are they as luminous as our sun, 

 or more so, or less so? Or are they unequal? 



