22 CARNEGIE INSTITUTION 



of various orders of brightness, also makes us aware that there are 

 clusters of stars which point unmistakably to the operation of law. 

 The vast aggregation of clusters making up the nebulous belt known 

 as the Milky Way, which spans the sky as a kind of celestial equator, 

 suggests orderty arrangement. 



When we assist our researches with the telescope, merely as an 

 optical appliance, evidences of such arrangement multiply until we 

 are finally led to conjecture, by statistical methods alone, that the 

 earth is situated within a vast cluster of stars verj' much more ex- 

 tended in the direction of the Milky Way than in other directions. 



If we fortify our telescope with means of measurement we shall 

 discover, after sufficient lapse of time, that some of the stars are in 

 motion relatively to others. As we persevere in our measurements 

 we shall discover an ever increasing number of stars partaking of 

 this motion ; and we shall finally conclude that all the stars are in 

 motion, some in one direction, and others in another. Some of these 

 motions are only apparent. Disentangling these, after immense 

 effort, we shall be able to recognize a peculiar drift of the stars, 

 precisely like that which appears in surrounding objects when we 

 are moving rapidly among them. This will finally prove to us that 

 the sun is only a star, and that it is in rapid motion like all the 

 others. Later we shall notice that separate groups of stars seem to 

 be moving in a common direction like swarms of meteors ; and we 

 shall begin to suspect that other evidences of law in these motions 

 may be revealed to us at any moment if we persevere in our in- 

 vestigations. 



Parallel with the discovery of these facts we shall be learning- 

 something of the distances of the stars. We are oppressed with the 

 conviction that, so long as we are studying the arrangement and 

 motion of the stars as objects upon a map, our information will be 

 lacking in a vital element. We long to gain some conception of the 

 space relations of the stars. From the very first we will have been 

 led to suspect that some stars are brighter than others because 

 they are nearer. When we attempt to test this hypothesis by 

 measurement, we shall encounter the most formidable difficulties. 

 Putting this difficulty in its simplest form it will mean that, if the 

 vast area of the earth's orbit, more than one hundred and eighty 

 million miles in diameter, were to be brightly illuminated and re- 

 moved to the distance of the nearer stars it would appear as a mere 

 point in the most powerful telescopes ; and to discover its dimensions 

 would require the greatest refinement of skill even in the more favor- 



