NOMOS. 155 



and c and d are within the surface. In this way, 

 then, it is possible to account for the existence of a 

 luminous eccentric focus within the comet, and also 

 for certain marked changes in the appearance of that 

 focus. If, for example, the focus d coincides with a 

 layer of the comet as it does in the dia- 



* The focus 



gram, we may very well suppose that the before men- 



.n i , n i tioned may 



effect will be to convert the part oi the undergo the 

 layer upon which it impinges into a jet 

 of luminous vapour. We may suppose, 

 moreover, that this jet will pass off in the der s- 

 direction in which it meets with least resistance, and 

 that this direction will not be constant, for as the 

 comet revolves upon its axis new jets will be formed 

 and other paths opened for them. All this is allow- 

 able, but is it probable ? It is certainly not altoge- 

 ther conjectural, as we shall see presently. 



In the same way it is possible to account for the 

 formation of the tail, for all that is necessary is to 

 suppose that the focal action of the sun's 



rr > B The conse- 



rays within the comet is to produce ex- quenceofthe 



. _ _. , focal concen- 



pansion of the parts corresponding to the tration of the 

 focus, and that the overlying layer or 

 layers are forced outwards from the side 



most removed from the sun, forced out- overlying 



substance of 



wards in this direction because the posi- the comet 



, , . into the tail. 



tion oi the locus must be nearer to this 

 side than to the side nearest to the sun ; and forced 

 outwards to an amazing extent, and with amazing 

 velocity, for the degree to which and the rate at 

 which a body of such extreme rarity as a comet may 

 expand under high degrees of heat, such as must be 



