272 FRAGMENTS OF SCIENCE. 



Under certain circumstances, it may be quenched by a 

 Nicol's prism, and we then obtain the true color of the grass 

 and foliage. Trees and meadows thus regarded exhibit a 

 richness and softness of tint which they never show as long 

 as the superficial light is permitted to mingle with the true 

 interior emission. The needles of the pines show this effect 

 very well, large-leaved trees still better ; while a glimmer- 

 ing field of maize exhibits the most extraordinary variations 

 when looked at through the rotating Nicol. 



Thoughts and questions like those here referred to took 

 me, in August, 1869, to the top of the Aletschhorn. The 

 effects described in the foregoing paragraphs were for the 

 most part reproduced in the summit of the mountain. I 

 scanned the whole of the sky with my Nicol. Both alone 

 and in conjunction with the selenite it pronounced the per- 

 pendicular to the solar beams to be the direction of maxi- 

 mum polarization. But at no portion of the firmament was 

 the polarization complete. The artificial sky produced in 

 the experiments recorded in the preceding discourse could, 

 in this respect, be rendered 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-lustre 

 hue which, in the case of the firmament, outlived the ex- 

 tinction of the brilliance. With certain substances, how- 

 ever, artificially treated, this dull residue may also be ob- 

 tained. 



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. 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 alternate extinction and restoration of the light imme- 

 diate. When this was done along the arc to which I have 



