PROGRESS OF MODERN ASTRONOMY 



of rings, but the inner ring is certainly transparent, 

 for the limb of Saturn has been observed through it. 

 It is also certain that though the space occupied by 

 the ring is transparent, it is not through the material 

 parts of it that the limb of Saturn is seen, for his limb 

 was observed without distortion; which shows that 

 there was no refraction, and, therefore, that the rays 

 did not pass through a medium at all, but between the 

 solar or liquid particles of which the ring is composed. 

 Here, then, we have an optical argument in favor of 

 the theory of independent particles as the material of 

 the rings. The two outer rings may be of the same 

 nature, but not so exceedingly rare that a ray of light 

 can pass through their whole thickness without en- 

 countering one of the particles. 



" Finally, the two outer rings have been observed for 

 two hundred years, and it appears, from the careful 

 analysis of all the observations of M. Struve", that the 

 second ring is broader than when first observed, and 

 that its inner edge is nearer the planet than formerly. 

 The inner ring also is suspected to' be approaching 

 the planet ever since its discovery in 1850. These ap- 

 pearances seem to indicate the same slow progress of 

 the rings towards separation which we found to be the 

 result of theory, and the remark that the inner edge 

 of the inner ring is more distinct seems to indicate that 

 the approach towards the planet is less rapid near the 

 edge, as we had reason to conjecture. As to the ap- 

 parent unchangeableness of the exterior diameter of 

 the outer ring, we must remember that the outer rin^s 

 are certainly far more dense than the inner one, and 

 that a small change in the outer rings must balance a 



47 



