154 NOMOS. 



crust ? After what has been said already, this is a na- 

 tural view to take of the matter ; but there is one seri- 

 ous objection to it, and this is the fact that the density 

 of the comet is not sufficient to bring the solar rays 

 to a focus within the body of the comet. With a 

 body of such rarity, indeed, the position of this focus 

 must be far, very far, beyond the surface, for it re- 

 quires a density as high as that of rock-salt to make 

 the focus coincide with the surface. There is, how- 

 ever, one way of getting over this difficulty, and this 

 is by supposing the comet to be composed of several 

 concentric layers with wide intervening spaces ; for 

 in this case each layer, as well as the whole comet, 

 will act as a spherical lens, each with its own focus, 

 and the foci of the innermost layers may be within 

 the outermost layers. This may be seen by a dia- 

 gram in which the rays proceeding from the sun, s, 

 are refracted by a comet, c, consisting of four sepa- 



FlG. 30. 



rate layers, and brought to a focus at four different 

 points, a b c and d respectively, of which points a 

 is beyond the surface, b coincides with the surface, 



