546 THE POPULAR SCIENCE MONTHLY. 



they send us inconsidera'ble, for the total effulgence of the stars 

 down to the 9i light-magnitude is equal to one eightieth part of 

 the effulgence of a full moon in a clear sky. "What light we get 

 from the stars of lower magnitude it is difficult to say, but it is 

 clear that the stellar world is not boundless, for were it so the 

 light from the infinite hosts of more and more remote suns would, 

 as Miss Gierke says, fill the sky with an indefinitely intense ra- 

 diance. It must, however, be remembered that it is not known 

 whether the undulations which cause light are capable of infinite 

 propagation. Nor, it may be added, can one be certain that the 

 mass of ether in which our cosmos swims is the only one in space ; 

 or, if space and ether be taken as convertible terms, that it is the 

 only mass differentiated — coarsened, so to speak, into a condition 

 fit for the evolution of matter and energy, and of the suns and 

 solar systems thus brought into being. The stars are arranged 

 according to their light-magnitudes, to each magnitude the nu- 

 merical value 2i being assigned, for mathematical reasons that 

 can not be here explained. Altair and Aldebaran are, strictly 

 speaking, the only stars of the first magnitude, and the light of 

 either of them would equal that of one hundred stars of the sixth 

 and one million stars of the sixteenth magnitude. Sirius, how- 

 ever, is nine times as bright as Aldebaran, and its magnitude 

 accordingly is expressed by the value — 1'4. Among the suns visi- 

 ble to us, it comes next to our own sun, whose magnitude is reck- 

 oned at — 25"-4 ; in other words, the sun is (to our earth) between 

 three and four million times as luminous as the Dog-star. The 

 most accurate photometric measures of the stars are now made 

 by the aid of photography, and the astronomers of a thousand 

 years hence will have before them exact light-histories of nearly 

 all the millions of stars of which the delicate and tireless gelatin 

 films can seize and retain the faintest light-impressions. To what 

 undreamed-of knowledge of our cosmos this wealth of accurate 

 records will lead ! 



One of the most important results of stellar photometry is the 

 aid it affords toward determining the distances of the stars. The 

 mean distance of stars of the same magnitude is approximately 

 the same ; and if, therefore, the distances of some of the nearer 

 stars are obtained, the approximate remoteness of any given cate- 

 gory is easily calculated. But to find independently the distance 

 of any individual star, its parallax must be known — the angle, 

 that is, between two lines drawn from the ends of a base-line of 

 known length to the star in question. Now, if the mean distance 

 between earth and sun be taken as such base-line, 93,000,000 miles 

 in length, to include an angle of one second (one 324,000th of a right 

 angle), the line must be drawn to an object 206,205 X 93,000,000 

 miles distant. "Well, no star is so near as this. The nearest star. 



