THE SKY OF THE ALPS 139 



the alternative extinction and restoration of the light im- 

 mediate. When this was done along the arc to which I 

 have referred, the alternations of light and darkness re- 

 sembled the play of sheet lightning behind the mountains. 

 There was an element of awe connected with the sudden- 

 ness with which the mighty masses, ranged along the line 

 referred to, changed their aspect and definition under the 

 operation of the prism. 



