170 LIGHT SCIENCE FOR LEISURE HOURS. 



mations which were referred to in our paper on 

 Comet I. 



Newton's hypothesis respecting comets' tails was 

 somewhat different. He supposed that the intensely 

 heated comet communicated its heat to the surrounding 

 ether, which thus grew rarer and ascended in the solar 

 atmosphere that is, flowed away from the sun pre- 

 cisely as heated air ascends from the earth. The ether 

 thus displaced would carry away with it the rarer por- 

 tions of the comet's substance, just as smoke is carried 

 upwards by a current of heated air. 



It will be seen at once that Newton's theory^ like 

 Kepler's, affords no explanation of lateral tails, or of tails 

 turned towards the sun. 



In modern times a theory has been founded on the 

 supposition that cometic phenomena may be due to 

 electrical agency. The German astronomer Olbers was 

 one of the first to propound this view, and many emi- 

 nent astronomers amongst others the younger Herschel 

 have looked with favour upon the theory. As yet, 

 however, we do not know enough respecting electricity 

 to accept with confidence any theory of comets founded 

 upon its agency. 



The comet respecting which I now have to treat 

 was discovered in the middle of June 1868, by Win- 

 necke. At first it was a telescopic object, but it 

 gradually increased in brilliancy until it became visible 

 to the unaided eye. In the telescope, at the end of 

 June, the comet appeared as a circular cloud rather 

 brighter in the middle, where there was a roundish spot 



