1 68 LIGHT SCIENCE FOR LEISURE HOURS. 



comet into invisible vapour. This action progressing 

 from without inwards, of course produces an apparent 

 diminution of volume. The diminution continues as 

 long as the comet is approaching the sun, and for yet 

 a few days after perihelion passage ; but soon after the 

 comet has begun to leave the sun's neighbourhood the 

 transparent vapour begins to return to its original 

 condition, the solar action being insufficient to keep 

 the whole of the vaporised matter in the gaseous state. 

 Thus the comet gradually resumes its original apparent 

 dimensions. 



There are few phenomena which have given rise to 

 more speculation than those presented by the tails of 

 comets. Astronomers who, in dealing with other 

 matters, have exhibited the soundest judgment, and 

 the most logical accuracy of argument, seem to feel free 

 to indulge in the most fancifuX speculations when deal- 

 ing with this subject. 



A favourite theory with the earlier astronomers was 

 founded on the observed peculiarity that the tails of 

 comets are usually turned directly from the sun. It was 

 supposed that the tail is not a really existent entity, 

 but merely indicates the passage of the solar rays 

 through space, after their condensation by the spherical 

 head of the comet. Just as a light received into a dark 

 room through a small aperture appears as a long ray 

 extending in a straight line through the room, so, 

 according to this theory, the sun's light, concentrated 

 by the comet's head, throws a long luminous beam into 

 space. Unfortunately for this view there is a want of 



