130 FKAGMENTS OF SCIENCE. 



But at no portion of the firmament was the polarisation 

 complete. The artificial sky produced in the experi- 

 ments recorded in the preceding pages could, in this 

 respect, be rendered far more perfect than the natural 

 one ; while the gorgeous 6 residual blue ' which makes 

 its appearance when the polarisation of the artificial 

 sky ceases to be perfect, was strongly contrasted with 

 the lack-lustre hue which, in the case of the firmament, 

 outlived the extinction of the brilliancy. With certain 

 substances, however, artificially treated, this dull residue 

 may also be obtained. 



All along the arc from the Matterhorn to Mont 

 Blanc the light of the sky immediately above the 

 mountains was powerfully acted upon by the Nicol. 

 In some cases the variations of intensity were astonish- 

 ing. I have already said that a little practice enables 

 the observer to shift the Nicol from one position to 

 another so rapidly as to render the alternative ex- 

 tinction and restoration of the light immediate. 

 When this was done along the arc to which I have 

 referred, the alternations of light and darkness resembled 

 the play of sheet lightning behind the mountains. 

 There was an element of awe connected with the sud- 

 denness with which the mighty masses, ranged along 

 the line referred to, changed their aspect and definition 

 under the operation of the prism. 



The physical reason of the blueness of both natural 

 and artificial skies is, I trust, correctly given in the 

 essay on the Scientific use of the Imagination published 

 in the second volume of these Fragments. 



