160 



rats the mist already precipitated, and the further, there- 

 fore, will be the point at which the mist is cleared from 

 that at which it was condensed, thus accounting for the 

 retrograde curvature of the posterior edge of the appendage, 

 and for the excess of this curvature over that of the opposite 

 side. 



The angular separation of the front and rear edges of the 

 tail will clearly be regulated, amongst other things, by the 

 angular capacity of the shadow in which that tail is formed, 

 which increases with the comet's proximity to the sun. 



Accordingly we should expect this angular separation to 

 be at its greatest in perihelio, which as a matter of fact has 

 been observed to be the case. Particular attention was 

 called to this phenomenon in the instance of Donati's comet 

 in 1858, and beautiful plates illustrative of it are given 

 in the 30th volume of the Astronomical Society's memoirs 

 by Prof Challis and Mr. Warren De la Rue. 



The fact that the maximum length and splendour of a 

 comet's tail is attained not at but after the passage of the 

 perihelion is only what we might reasonably expect, for, 

 as we know, time is required in which to produce any 

 physical change, and consequently that augmentation of the 

 cometary atmosphere resulting from the heat received in 

 perilielio must necessarily be produced some time after that 

 heat has been received, and therefore after the perihelion 

 passage. 



The diminution in size which the nucleus of a comet 

 undergoes as it approaches the sun, and the subsequent 

 expansion which takes place as it recedes from it, a diminu- 

 tion and expansion which are contemporaneous with, but 

 reversed in order to, the dilation and contraction of the 



