8 A Century of Science 



eye. It may suffice to observe that we thus catch 

 the fleeting aspects of sun-spots and preserve them 

 for study ; we detect the feeble self -luminosity still 

 left in such a slowly cooling planet as Jupiter ; 

 and since the metallic plate does not quickly 

 weary, like the human retina, the cumulative ef- 

 fects of its long exposure reveal the existence of 

 countless stars and nebulaB too remote to be other- 

 wise reached by any visual process. By such 

 photographic methods George Darwin has caught 

 an equatorial ring in the act of detachment from 

 its parent nebula, and the successive phases of the 

 slow process may be watched and recorded by gen- 

 erations of mortals yet to come. 



To appreciate the philosophic bearings of this 

 vast enlargement of the mental horizon, let us re- 

 call just what happened when Newton first took 

 the leap from earth into the celestial spaces by 

 establishing a law of physics to which moon and 

 apple alike conform. It was the first step, and 

 a very long one, toward proving that the terrestrial 

 and celestial worlds are dynamically akin, that the 

 same kind of order prevails through both alike, 

 that both are parts of one cosmic whole. So late 

 as Kepler's time, it was possible to argue that the 

 planets are propelled in their elliptic orbits by 

 forces quite unlike any that are disclosed by purely 



