﻿460 On some Spectrum Observations of Comets, 



The invisible spaces between the envelopes may possibly cor- 

 respond to a condition of the vapour too cool to emit light, and 

 yet not condensed so as to reflect light. 



The exterior parts of the coma and the tail, which have been 

 found to be polarized in a plane, showing the light to come from 

 the sun, may be supposed to consist of the vapour of the nu- 

 cleus condensed into widely scattered particles of great mi- 

 nuteness. 



The remarkable phenomenon of the great rapidity with which 

 the tail is seen to extend itself to enormous distances in a direc- 

 tion from the sun remains unexplained. It may be suggested 

 that the instant at which the matter appears to come under the 

 influence of repulsion from the sun, may be that at which the 

 vapour is condensed into discrete particles, and be in some way 

 connected therewith*. 



Many years since Benedict Prevotf suggested the following 

 hypothesis. The head of the comet by the sun's heat is con- 

 verted into vapour which is invisible, and expands to a great dis- 

 tance from the head in all directions. Behind the head, where 

 the vapour is sheltered from the sun's heat, it is condensed into 

 cloud, which reflects light and appears as the tail. This cloud 

 passes back into invisible vapour as, by the comet's motion, it 

 becomes exposed again to the solar beams. 



This theory is obviously inconsistent with the observed ap- 

 pearances and forms of the tails, and especially with the rays 

 which are frequently projected in a direction different from that 

 of the tail, with the absence of tail immediately behind the head, 

 and with the different degrees of brightness of the sides of the 

 tail. 



I should not refer to this almost-forgotten hypothesis, but 

 for the circumstance that the same theory essentially forms part 

 of the recent ingenious speculations of Professor Tyndall on the 

 nature of comets, with the difference, however, that clouds 

 formed in the cool shade of the comet's head are supposed by 

 him to be due to chemical action which can then take place, and 

 not, as in Prevot's theoiy, to a lower temperature alone. 



For further positive knowledge of the nature of cometary 

 phenomena we must doubtless wait until the searching method 

 of analysis by the prism can be applied to the series of changes 

 presented by a brilliant comet. 



* Phil. Trans. 1868, p. 563. 



t Arago, ' Popular Astronomy/ translated by Smyth and Grant, vol. i. 

 p. 623. 



