ARTIFICIAL SKY. 129 



as the light which falls upon the landscape ; the other 

 part reaches us from a certain depth within the objects 

 which compose the landscape, and it is this portion of 

 the total light which gives these objects their distinc- 

 tive colours. The white light of the sun enters all 

 substances to a certain depth, and is partly ejected by 

 internal reflection ; each distinct substance absorbing 

 and reflecting the ligbt, in accordance with the laws of 

 its own molecular constitution. Thus the solar light is 

 sifted by the landscape, which appears in such colours 

 and variations of colour as, after the sifting process, 

 reach the observer's eye. Thus the bright green of 

 grass, or the darker colour of the pine, never comes to 

 us alone, but is always mingled with an amount of light 

 derived from superficial reflection. A certain hard 

 brilliancy is conferred upon the woods and meadows by 

 this superficially-reflected light. Under certain cir- 

 cumstances, it may be quenched by a Nicol's prism, and 

 we then obtain the true colour 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 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 Aletsch- 

 horn. The effects described in the foregoing para- 

 graphs 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 polarisation- 



