150 LIGHT SCIENCE FOR LEISURE HOURS. 



of Halley's comet, and he found that the place of the 

 star was not affected to an appreciable extent. ' 



Whether the nucleus of a comet is solid or not had 

 long been a disputed point among astronomers. With 

 telescopes of moderate power the bright point within 

 the coma presents an appearance of solidity which 

 might readily deceive the observer. But with an 

 increase of power the nucleus assumes a different 

 appearance. Instead of having a well-defined outline 

 it appears to merge into the coma by a somewhat rapid 

 gradation but not by an abrupt variation of light. 

 Grood observers have reported the extinction of tele- 

 scopic stars behind the nuclei of comets, but there are 

 peculiar difficulties about an observation of this sort ; 

 and it is very difficult to determine whether a star is 

 really concealed from view by the interposition of the 

 nucleus or simply obliterated by the glare of light. 



The tail of a comet appears nearly always as an 

 extension from the coma, and a dark interval is usually 

 seen between the head and the tail. But there is an 

 immense variety in the configuration of comets' tails. 

 The comet of 1744 had six tails spread out like a fan. 

 The comet of 1807 had two tails both turned from 

 the sun. The comet of 1823 had also two tails, but 

 one was turned almost directly towards the sun. Other 

 comets have had lateral tails. 



The processes which seem to be passed through by 

 comets during their approach towards and recession 

 from the sun have proved very perplexing to astrono- 

 mers and physicists. When first seen a comet usually 



