THE LAND OF AZURE BLUE 53 



rays come through. Such a body is blue copperas, or 

 sulphate of copper ; another is methyl blue, one of the 

 aniline dyes ; another is pure water, which gives only 

 a slight advantage to the blue rays, so that the light 

 must pass through a thickness of 30 feet or more before 

 the blue tint is obvious. Thus, part of the blueness of 

 the Cote d'Azur is accounted for namely, the blueness 

 of the sea when the sunlight is strong and is reflected 

 from the white rocks and sand lying 30 feet to 100 feet 

 below the surface of the water. 



There are, of course, other self-coloured transparent 

 bodies which allow only rays of one colour to pass. 

 Thus, blood -red, or haemoglobin, the pigment of the 

 blood, allows chiefly red rays to pass through it. Yellow 

 rays only pass through a solution of saffron or of chromic 

 acid ; green only or chiefly through green copperas (sul- 

 phate of iron) or through leaf-green or chlorophyll. Colour 

 is very generally due in natural objects to such transparent 

 bodies which absorb or stop all the coloured rays of light 

 as it passes through them, excepting those of one tint 

 or, to be more correct, nearly all except those of one tint. 



But the blue of the blue frog and a great deal of the 

 blue in nature is due to another cause. If you are a 

 smoker, or the friend of a smoker, watch the fine curling 

 lines of smoke ascending from a cigar when it is being 

 consumed in bright sunshine. You will see that it has 

 a blue, even an azure blue, tint as the sunlight falls upon 

 it. But if you let the smoke get between the sun and 

 your eyes you will notice that the little curling clouds 

 are no longer blue, but reddish-brown, in appearance. 

 The smoke is not a transparent blue ; looked at as a 

 transparent body, it is brown ! Further, when the smoke 

 has passed into the smoker's mouth and is ejected after 

 remaining there for a few seconds, the cloud no longer 

 looks blue, even when the sunlight falls on it and is 



