THE TWO COMETS OF THE YEAR 1868. 169 



analogy between the two cases thus brought into com- 

 parison. The light shining into a room produces the 

 appearance of a ray, because it illuminates the air and 

 the small particles of floating dust which it encounters 

 in its passage. There is nothing corresponding to this 

 in the interplanetary spaces. If there were, the sky 

 would never appear black, since the sun would always 

 be shining on matter capable of reflecting his rays* 



Kepler was the first to form a reasonable hypothesis 

 respecting comets' tails. He supposed that the action of 

 the solar heat dissipates and breaks up a comet's substance. 

 The rarer portions are continually swept away, he 

 imagined, by the propulsive energy of the solar rays, 

 and are swept in this way to enormous distances from 

 the comet's tail. The denser portions remain around 

 the nucleus and form the coma. 



The modern theory respecting light (according to 

 which there is no propulsion of matter from the sun, 

 but a simple propagation of wave-like motion), does not 

 affect Kepler's hypothesis so much as might be ima- 

 gined. Whatever theory of light we adopt we are 

 forced to assume an extreme tenuity in the matter 

 which forms the tails of comets. And when once we 

 have made this assumption, we are enabled to admit 

 that even the propagation of a wave-like motion through 

 the ether which is supposed to occupy the interplanetary 

 spaces, might suffice to carry off the attenuated nebu- 

 lous matter with tremendous rapidity. 



The defect of Kepler's theory is that it appears 

 insufficient to account for those anomalous tail-for- 



