98 FRAGMENTS 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 entitled 
"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.] 
CHAPTER V. 
THE SKY.* 
INVITED 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 1?i6 Forum, February, 1&J8, 
