1882.] 



on Comets. 



9 



have not only to consider the effect of perspective, but also that the 

 comet itself is advancing, so that the visible tail is due to the portion 

 of space which at the time contains all the repelled matter, each 

 particle describing its own independent orbit, and reflecting to the 

 eye the solar light or giving out its own light, as the case may be.* 

 The value of the repulsive force which would be necessary on this 



theory has been investigated by Bessel, Peirce, and others.f Recently 

 Bredichin J has investigated the curvatures of the tails of a number 

 of comets. According to him, they fall into three classes, which are 

 represented in this diagram, each type of curve depending upon a 

 different assumed value of the repulsive force. This leads to another 

 point, namely, the secondary tails which are often present. Some 

 of these appear to be darted off with an energy of repulsion so enor- 



* As a rule, the tails of comets appear to be luminous by reflected solar light, 

 but at times the stuff which emits the light giving a spectrum of bright bands 

 is carried into the tail to a greater or less distance from the head, 

 t See numerous papers by Faye in the ' Comptes Kendus.' 

 X ' Annales de rObservatoire de Moscou,' vol. v. liv. 2, p. 30 ; and * Astr. 

 Nachr.'No. 2411. 



