THE NUMBER OF THE STAUS. 



105 



last mentioned, i.e., the combined brightness of three candles is 

 not greater than that of two in so high a proportion as the 

 brightness of two candles is greater than that of one. To 

 obtain the same ijroportionate increase in the brightness of the 

 light, we mnst donble the number of candles ; we must add 

 two to two, just as we added one to one ; and if we wish to 

 carry on the process, to obtain the same proportionate increase 

 in brightness once again, we should have to add four additional 

 candles to the existing four. 



So it is with the stars ; an average star of the 2nd magni- 

 tude gives less than half the light of an average first magnitude 

 star ; or, in general, two average stars of one magnitude are 

 about equal in light to five average stars of the next. More 

 precisely, a difference of five magnitudes in the light of a star, 

 corresponds to a diminution of light in the ratio of 1 to 100 ; 

 and a typical star of the 6th magnitude gives only one-hundredth 

 the amount of light of a typical star of the 1st magnitude. In 

 order, therefore, to bring fainter stars down to the 11th magni- 

 tude just within the range of visibility of a telescope, we must 

 increase the beam of light entering the eye one hundred times ; 

 so that the telescope must have an object glass one hundred 

 times the area of the pupil of the eye. In other words it must 

 be between two and three inches in diameter. In order to 

 reach to stars five magnitudes fainter still, that is to say to 

 render visible stars of the 16th magnitude, the telescope must 

 have an aperture of 28 inches in diameter. Upon this com- 

 putation, the largest telescope in the world, which is five feet 

 in diameter, will just show stars of between the 17th and 18th 

 magnitude. This is the chief instrument of the Mount "Wilson 

 Observatory in California. A still larger telescope, now being- 

 built, which in its turn will be tlie largest telescope in the 

 w^orld, will have a diameter of 100 inches, but this will only 

 enable stars to be seen about 1^ magnitudes fainter than those 

 visible by means of the great five-foot telescope, now at Mount 

 Wilson. For, when telescopes of dimensions like these are 

 reached, a very great further increase in size is required to 

 obtain only a very small increase in its penetrating powder ; an 

 immense increase in the size of the instrument, in its cost, and 

 in the difficulty of its manufacture and manipulation, enables 

 the observer to go but a very little way further down the scale 

 of faint stars. 



But, in the investigation of faint stars, astronomers are 

 fortunately not confined to those that they can see, even with 

 the aid of a great telescope, because here photography comes to 



