110 A CENTURY'S PROGRESS IN ASTRONOMY. 



so closely crowded together as to present the ap- 

 pearance of a continuous whole. Clerk-Maxwell's 

 explanation which had been suggested by the 

 younger Cassini in 1715, and by Thomas Wright 

 in 1750 was at once adopted, and has since 

 been proved by observation. In 1888 Hugo 

 Seeliger (born 1849), director of the Munich 

 Observatory, showed from photometric observa- 

 tions the correctness of the satellite -theory; 

 while Barnard in 1889 witnessed an eclipse of 

 the satellite Japetus by the dusky ring. The 

 satellite did not disappear, but was seen with 

 perfect distinctness. The final demonstration of 

 the meteoric nature of the rings was made by 

 Keeler at the Alleghany Observatory in 1895, 

 with the aid of the spectroscope. By means 

 of Doppler's principle, he found that the inner 

 edge of the ring revolved in a much shorter 

 time than the outer, proving conclusively that 

 they could not be solid. This was confirmed by 

 the observations of Campbell at Mount Hamilton, 

 Henri Deslandres at Meudon, and Belopolsky 

 at Pulkowa. 



In 1851 a startling theory regarding Saturn's 

 rings was put forward by the famous Otto 

 Wilhelm von Struve (1819-1905). Comparing his 

 measurements on the rings made at Pulkowa in 

 1850 and 1851 with those of other astronomers 



