THE EVER- WIDENING WORLD OF STARS. 43 



which lie beyond the sphere of those visible to us. 

 But it was reserved for Sir W. Herschel to apply exact 

 observations to the unseen star- systems. He literally 

 gauged the celestial depths. With a telescope whose 

 light-gathering power extended the range of vision to 

 about eight hundred times its natural limit, he swept 

 the whole of the northern heavens. He estimated the 

 depth of the system of stars in every direction by a 

 simple and natural process. For, like all great thinkers, 

 he struck out modes of inquiry which, the moment they 

 were presented to the world, seemed so obvious, that 

 the wonder was how they could have remained so long 

 undetected. He said that precisely as the quantity of 

 water passed through by the sailor s lead-line marks 

 the depth of the sea, so the number of stars which can 

 be seen when a telescope of given power is turned 

 towards any part of the heavens is a measure of the 

 depth of the sidereal system in that direction. In in 

 dividual cases, indeed, the law may not be true, just as 

 the sailor s lead-line may light on the peak of some 

 sunken rock, and so give no true measure of the general 

 depth of the sea in the neighbourhood. But when the 

 average of a great number of such star-gaugings is 

 taken, then we may feel tolerably certain that on 

 applying the simple rule devised by Herschel we shall 

 form no inaccurate estimates of our system s extent in 

 any direction. 



Thence arose his great theory of the stellar system. 

 He showed that our sun is but one of an immense 

 number of suns, distributed in a region of space resem- 



