532 A Short History of Astronomy [CH. XII. 



he began to study the sidereal system per se and the 

 mutual relations of its members. From this point of view 

 the sun, with its attendant planets, became one of an 

 innumerable host of stars, which happened to have received 

 a fictitious importance from the accident that we inhabited 

 one member of its system. 



258. A complete knowledge of the positions in space 

 of the stars would of course follow from the measurement 

 of the parallax (chapter vi., 129 and chapter x., 207) of 

 each. The failure of such astronomers as Bradley to get the 

 parallax of any one star was enough to shew the hopelessness 

 of this general undertaking, and, although Herschel did make 

 an attack on the parallax problem ( 263), he saw that the 

 question of stellar distribution in space, if to be answered 

 at all, required some simpler if less reliable method capable 

 of application on a large scale. 



Accordingly he devised (1784) his method of star- 

 gauging. The most superficial view of the sky shews that 

 the stars visible to the naked eye are very unequally dis- 

 tributed' on the celestial sphere ; the same is true when 

 the fainter stars visible in a telescope are taken into account. 

 If two portions of the sky of the same apparent or angular 

 magnitude are compared, it may be found that the first 

 contains many times as many stars as the second. If we 

 realise that the stars are not actually on a sphere but are 

 scattered through space at different distances from us, 

 we can explain this inequality of distribution on the sky 

 as due to either a real inequality of distribution in space, 

 or to a difference in the distance to which the sidereal 

 system extends in the directions in which the two sets of 

 stars lie. The first region on the sky may correspond to 

 a region of space in which the stars are really clustered 

 together, or may represent a direction in which the sidereal 

 system extends to a greater distance, so that the accumula- 

 tion of layer after layer of stars lying behind one another 

 produces the apparent density of distribution. In the' same 

 way, if we are standing in a wood and the wood appears 

 less thick in one direction than in another, it may be 

 because the trees are really more thinly planted there or 

 because in that direction the edge of the wood is nearer. 



In the absence of any a priori knowledge of the actual 



