THE NUMBER OF THE STARS. 



103 



call " the stars of the microscope " seem, according to what 'men ; of 

 science tell us, to be as numerous and as wonderful as the stars of 

 the heaven above. And it is yet more extraordinary that the very 

 movements of the atoms are like those of the solar system. I will 

 not further encroach upon the time of the Lecturer, who has so 

 much to bring before our notice, but I will now ask Dr. Chapman 

 to address us on " The Xumber of the Stars." 



THE XUMBER OF THE STARS. By Sydney Chapman, 

 B.A., D.Sc, F.R.A.S., Chief Assistant at the Eoyal 

 Observatory, Greenwich. 



rriHE suljject of my lecture is the Number of the Stars, a 

 JL subject which I might almost say did not exist in ancient 

 times, Ijecause it \vas then very generally believed that the 

 stars had no number : they were innumerable, infinite in 

 number. In the Bible, for example, we frequently find the 

 number of the stars classed \vith the sands on the seashore, as 

 an expression for a " multitude which no man can number." 

 We shall see later on that this metaphor does but scant justice 

 to the sands of the seashore, for the number of the stars is 

 really very much smaller than that of the grains of sand. 



As we look up to the sky on a clear night, we see a numl)er 

 of stars so great that probably no one in this room has ever 

 thought of trying to count them. The number visible varies 

 with the clearness of the sky and the keenness of the vision of 

 the beholder, so that different observers on different nights and 

 from different stations see different numbers of stars. But tlie 

 stars visible, separately and discretely, to even the keenest 

 sight and on the clearest night are not nearly so numerous as 

 might be supposed : their number is very limited. But since 

 the stars vary much in their apparent brightness, so that they 

 range from the brightness of Sirius down to those so faint that 

 they are just on tlie limit of our vision, it is reasonable to 

 suppose that there are stars fainter still, wdiich, if we had better 

 eyesight, we might be able to observe. Tliat indeed is the 

 case ; the stars which we can see with our unaided sight have 

 traditionally been divided into classes according to their 

 brightness, classes which are known technically as " magnitudes." 

 Of these magnitudes, there are six for stars within the range of 

 our unassisted sight. The stars of any one class or magnitude 

 are not quite equal in . brightness, some being brighter and 



