ICO 
rate 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 
perihelio 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 
