176 HISTOKY OF ASTKONOMY. 



space they occupy, would be least in the direction of a 

 visual ray perpendicular to the lamina, and greatest in 

 that of its breadth ; increasing rapidly in passing from 

 one to the other direction, just as we see a slight haze in 

 the atmosphere thickening into a decided fog-bank near 

 the horizon, by the rapid increase of the mere length of 

 the visual ray. 



Herschel was conducted to this view of the Milky Way 

 by the following considerations. Supposing the stars to 

 be situated, in general, at equal distances from each other, 

 the number of stars observed in the field of a telescope 

 ought to be about the same in all possible directions, pro- 

 vided the stars extend in all directions^ to the same dis- 

 tance. But if we have a stratum of stars at equal dis- 

 tances from each other, of a form whose thickness is small 

 in comparison with its diameter, then the number of stars 

 visible in the different directions, will lead us to a knowl- 

 edge both of the exterior form of the starry stratum, and 

 of the place occupied by the observer. For example, if 

 within a certain circle of the heavens we count ten stars, 

 and in a circle of the same diameter, taken in a different 

 direction, we count eighty stars with the same telescope, 

 the lengths of the two visual rays will be in the ratio of 

 1 to 2, or the cube roots of 1 and 8. This is substantially 

 Herschel's method of star-gages, in which he employed a 



