ASTRONOMY 119 



or can we find stars scattered through space at infinite distance 

 in all directions? The peculiar phenomenon known as the 

 "Milky Way" seems to indicate that perhaps the stellar uni- 

 verse is limited to a huge, flat, disc-shaped space near the 

 center of which our system lies. If we look toward the flat 

 sides of this space we see comparatively few stars and these 

 are mostly bright ones and therefore presumably not very far 

 from us. But if we look toward the edges of the disc we see 

 very many stars, near and distant, and the more distant ones 

 seem so closely packed that to the naked eye they blend into a 

 uniform milky appearance. 



Now it can be shown that, if the stars did extend to 

 infinity in all directions, and there was nothing to intercept 

 their light, the whole sky would appear as a uniform blaze of 

 light. Since this is not the case, either there must be limits 

 to stellar space or else there must be something between us and 

 the stars which absorbs their light. Earlier in this lecture 

 I said that there were a large number of dark stars and 

 meteorites and also dust particles scattered through space, 

 and these of course must intercept and absorb some light. Of 

 course this "cosmic dust" is not at all thickly distributed, but 

 even if there were only one particle of it in each cubic mile, 

 the light of the very distant stars would be appreciably 

 weakened. It has recently been shown that there is some 

 evidence that such a weakening of the light of the farthest 

 stars really occurs. Whenever there exists an absorption of 

 light, there is also what we call dispersion, — that is, the light 

 waves of different colors do not travel equally fast. The 

 difference in velocity of the red and blue waves due to the 

 weak absorption would be far too small to detect in such a 

 relatively short distance as that from the sun to the earth, 

 about ninety million miles, but from a study of very distant 

 variable stars there seems to be some reason for believing 

 that blue light travels a little slower through space than red 

 light. 



