28 MR. J. SMITH ON THE ORIGIN OF COLOUR 



made available for the purpose. But the scientific explana- 

 tion, as we have seen^ rests completely ou Newton's theory 

 of light ; so that to object to it may appear to be, virtually, 

 objecting to Newton's theory of large and small molecules, 

 both of light and of matter, of molecules of the thickness 

 requisite for reflecting blue, red, green or any other colour. 

 This, however, I do not mean to attempt, as nothing is really 

 known of the size of molecules of any kind. Lengthened 

 observation has, however, led me to the conclusion that 

 the colour of the sky, and especially "the bright azure 

 which tinges the mountains of the distant landscape," can 

 be accounted for on a more easy and simple principle. I 

 have often remarked that it is not the most distant hills 

 that are the bluest. In a mountainous district the inter- 

 mediate are of a darker and more beautiful blue than the 

 more distant; and in a pure atmosphere when many 

 clouds are flying, the transparency of the air being great, 

 the nearer hills are of the deepest blue. 



45. There are two ways at least in which this may occur. 

 When the nearest clouds are low they not only throw sha- 

 dows on the hills, but they prevent many of the more per- 

 pendicular rays reflected from the hills from reaching the 

 eye. From this circumstance the hills appear in the 

 shade, but the more distant and horizontal rays being 

 reflected from the hills thus placed in the shade, give them 

 the blue colour which is so often described as the blue 

 colour of the air. I have watched until these clouds have 

 passed away, and until I saw the reflected rays grey and 

 brown from rock and heather. In like manner the shadow 

 cast by a single cloud and illuminated by a distant white 

 ray is frequently seen to be blue, and on a hill white with 

 snow the shadow of such a cloud, thus illuminated, becomes 

 very instructive. For we see at the same time on the snow 

 grey shadows and blue shadows ; proving that white seen 

 through black does not become blue, but that an insu- 



