74 PRESSURE OF LIGHT 



downwards. If this is what really happens with 

 comets, if the front or " envelope " on the sun side 

 of the nucleus is really the boundary to which the 

 fountains reach, it is possible to calculate the ratio 

 to gravitative pull of the repulsive force which first 

 destroys the forward velocity and then pushes the 

 matter back to form the tail. In some comets the 

 repulsion is forty, in some twenty times the pull, 

 while in some it is only slightly in excess. Some 

 comets have several tails with apparently a different 

 ratio of push to pull in each. In cases such as 

 these light-pressure might account for what is 

 observed if we could suppose that, as the comet 

 approaches the sun, the nucleus sends out clouds 

 of equal-sized dust, or, maybe, of equal-sized drop- 

 lets, and all with nearly the same velocity. But in 

 Morehouse's comet, 1908, Mr. Edington 1 finds that 

 the apparent paths of the streaming matter would 

 require repulsion hundreds of times greater than 

 the attraction, and it is hardly possible that the 

 pressure of light could account for this. 



The light-pressure theory, then, will not suffice to 

 explain the motions which are in some cases appa- 

 rently observed. But it is a fascinating theory, and 

 the only theory which seeks to explain the forma- 

 tion of the tails of comets on definite lines. Any 

 electrical explanation is at present vague ; and 

 though we may be almost sure that the self- 

 1 Monthly Notices R,A.S., March 1910, 



