Notes on Star- Streams. 5 



supposing that such a figure correctly represented the figure of 

 the section. He looked upon it as affording but a rough 

 indication of the true figure. He had indeed noticed, so early 

 as 1785, that there is a tendency in the Milky Way to cluster 

 around definite regions of the heavens ; and he saw that the 

 fact of such clustering was sufficient to account for many 

 irregularities of the figure, quite irrespectively of the absolute 

 extent of the Milky Way in space. If we are looking from a 

 height at the lights of a large town, we may fairly assume that 

 a row of many lights very closely ranged, lies at a greater 

 distance from us than another row containing lights more 

 widely dispersed, if we have reason to suppose that throughout 

 all the streets of the town the lights are separated by distances 

 approximately equal. But if we have reason to suspect that 

 there are some streets lighted more fully than others, the 

 inference would be no longer valid. And, again, Herschel 

 suspected that there are stars so large, as to bear a sort of 

 sway among other stars by superior attractive influence. 

 Here, then, was another element of difficulty, since it becomes 

 clear (1) that the brilliancy of a star is no positive evidence of 

 proximity ; and (2) that there may be (besides the obvious 

 clusterings already considered) laws of systematic distribution, 

 which might largely modify the evidence afforded by star- 

 gauging. For instance, returning to the illustration given 

 above, if we have reason to suspect that there are many lights 

 of superior brilliancy, in some parts of a town, and that further 

 there are in some streets laws of arrangement among the lights, 

 or that there are irregularities of surface-contour, which pro- 

 duce here and there a greater or less foreshortening than would 

 result on a level ground, we should have to make allowance for 

 these points in attempting to form an estimate of the distances 

 at which the different parts of the town are removed from us. 



Still, the results obtained by Sir W. Herschel have very 

 properly been accepted as affording general evidence of high 

 value. 



Sir J. Herschel, during his residence at the Cape of Grood 

 Hope, carried out an extensive series of observations of the 

 southern heavens. Applying his father's method of gauging, 

 with a telescope of equal power, he obtained a result agreeing, 

 in a most remarkable manner, with those obtained by Sir 

 William Herschel. It appeared, however, that the southern 

 hemisphere is somewhat richer in stars than the northern, a 

 result which has been accepted as indicating that our system is 

 probably somewhat nearer the southern than the northern part 

 of the galactic nebula. 



Combining the results obtained by the two Herschels, we 

 should assign to the stratum of stars a figure somewhat resem- 



