130 FKAGMENTS OF SCIENCE. 



selenite, it pronounced the perpendicular to the solar 

 beams to be the direction of maximum polarisation. 

 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 ' 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 moun- 

 tains was powerfully acted upon by the Nicol. In 

 some cases the variations of intensity were astonishing. 

 I have already said that a little practice enables the ob- 

 server to shift the Mcol from one position to another 

 so rapidly as to render the alternative extinction and 

 restoration of the light immediate. When this was 

 done along the arc to which I have referred, the al- 

 ternations of light and darkness resembled the play of 

 sheet lightning behind the mountains. There was an 

 element of awe connected with the suddenness with 

 which the mighty masses, ranged along the line referred 

 to, changed their aspect and definition under the opera- 

 tion of the prism. 



