NOMOS. 157 



is one point in the issue of this process of diminution 

 which demands our attention, and this is the trans- 

 formation of the comet from an irregular into a 

 regular form. Before the perihelion the comet is 

 furnished with a formidable tail ; after the perihelion, 

 it is a tail-less rounded body, surrounded with a 

 delicate halo. All the previous irregularity of form 

 is lost ; and how ? Is it not by the vaporisation of 

 an external layer or layers which had been raised 

 into irregular forms in the process of vaporisation, 

 and by the consequent exposure of another layer, 

 itself the outermost, perhaps, of several other layers? 

 Is not the very halo a sign either of a distinct con- 

 centric layer, or of the more delicate substance which 

 had intervened between the layer which we suppose 

 to have been dissipated, and the layer which now 

 constitutes the outer visible surface ? Again, is not 

 the dark stripe which divides the tail longitudinally 

 into two parts a revelation of the hollow space which 

 underlies the layer or layers which have been lifted 

 up to form the tail ? Assuredly, this dark stripe is 

 indicative of hollowness, and not of the shadow of 

 the comet, as was once supposed, for it exists when 

 the tail is turned so much aside as to be out of the 

 shadow of the body of the comet. 



It is not upon mere conjecture, therefore, that we 

 rest the existence of that peculiar laminated struc- 

 ture which has been assumed to exist in accounting 

 for the peculiar changes of Halley's comet. On the 

 contrary, there is a foundation of fact which will 

 bear the theory, a foundation which will appear the 

 more stable the more we are convinced that the 



