1908] 071 Recent Researches in the Structure of the Universe. 301 



Little Variation with Galactic Longitude. 



At the same distance from the Milky-way we shall find approximately 

 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 

 approximation will be already a tolerably close one. 



Heal 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 

 instance, 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 

 practically assumed ? And in this case are they as luminous as our 

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



If so, how many of them are brighter than our sun, how many 

 fainter ? Or, to be more particular, how many per cent, of the stars 

 are 10, 100, 1000, etc., times more luminous than our sun ? How 

 many are equal to the sun, or 10, 100 times fainter ? Or in two 

 words : What is the nature of the mixture ? or, lastly, what is the 

 mixture-latv of the system of the stars ? 



And furthermore : 



In travelling on, shall we find the stars in reality equally thickly, 

 or rather thinly, crowded everywhere ? Or shall w^e find that after 



