CHAPTERS ON THE STARS. 



233 



■evident by Fig. 1, where S represents the position of a star, re- 

 garded as a luminous point, while A and B are screens placed at such 

 a distance that each will receive the same amount of light from the 

 star. If the screen B is twice as far as the screen A, its sides must be 

 twice as large as those of A in order that it shall receive all the light 

 that would fall on A. In this case its surface will be four times the 

 surface of A. It is then evident that any small portion of the surface 

 of B will receive one fourth as much light as an equal portion of 

 surface A. Thus an eye or a telescope in the position B will receive 

 from the star one fourth as much light as in the position A, and the 

 star will seem one fourth as bright. 



The fact is, however, that the stars are very unequal in their actual 

 brightness, and in consequence the apparent magnitude of a star gives 

 us no clue to its distance. Among the nearer of the stars are some 

 scarcely, if at all. visible to the naked eye, while among the brighter 



ones are several whose distances are immeasurably great. A remark- 

 able example is that of Caropes, the second brightest star in the heavens. 



For these reasons astronomers are obliged to content themselves, 

 in the first place, with determinations of the actual amount of light 

 that the various stars send to us, or their apparent brilliancy, without 

 regard to their distance or actual brilliancy. The ancient astronomers 

 divided all the stars they could see into six classes, the number ex- 

 pressing the apparent brightness being called the magnitude of the star. 

 The brightest ones, numbering in all about fourteen, were said to be 

 of the first magnitude. The fifty next in brightness were said to be of 

 the second magnitude. Three times as many, an order fainter, were of 

 the third magnitude. The progression was continued up to the sixth 

 magnitude, which included those which were barely visible. 



As the stars are actually of every degree of apparent brilliancy, 

 no sharp line of demarkation could be drawn between those of one 

 magnitude and those of the magnitude next higher. Hence, different 



