98 VRA GMENTS OF SCIENCE. 



perpendicular to the solar beams to be the direction of 

 maximum polarization. But at no portion of the firma- 

 ment was the polarization complete. The artificial sky 

 produced in the experiments 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 polarization of the 

 artificial sky ceases to be perfect, was strongly contrasted 

 with the lack-luster hue which, in the case of the firma- 

 ment, 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 astonishing. 1 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 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 

 resembled the play of sheet lightning behind the moun- 

 tains. 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 operation of the prism. 



[In the last edition of the " Fragments of Science" an essay on 

 " Dust and Disease" followed here; but as almost all my writings on 

 the " Germ Theory" are now collected in a single volume entitle 4 

 "Essays on the Floating Matter of the Air," " Dust and Disease" no 

 longer appears in the " Fragments." In its place I venture to intro- 

 duce a short article written early last year for an important American 

 magazine. J 



CHAPTER V, 



THE SKY.* 



JNVITED to write for the Forum an article that 

 would have brought me face to face with " problems of 

 life and mind" for which I was at the moment unprepared, 

 and unwilling to decline a request so courteously made, I 



* From The Forum, February, 1888. 



