138 FRAGMENTS OF SCIENCE 



tain 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 emis- 

 sion. The needles of the pines show this effect very well, 

 large-leaved trees still better; while a glimmering 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 on 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 

 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 per- 

 fect 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 con- 

 trasted 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 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 



