78 SECRETS OF EARTH AND SEA 



and so-called " black " eyes. The blue colours in birds' 

 feathers and insects' wings are produced without blue pig- 

 ment by special effects of reflection, and where green is 

 the colour it is often due to the addition of a small 

 quantity of yellow pigment to what would otherwise look 

 blue: though some caterpillars and grasshoppers have a 

 real green pigment in their skin. Flowers, on the other 

 hand, have true soluble blue " pigments," and green ones 

 too, notably that called leaf-green or chlorophyll. The 

 little green tree frog has no blue or green pigment in its 

 skin ; only a yellow pigment. Sometimes rare specimens 

 are found in which the yellow pigment is absent altogether, 

 and then the little frog is turquoise-blue in colour. But 

 there is no blue pigment in the skin ; only a finely- 

 clouded translucent film overlying a dead-black deep 

 layer of the skin, and the result is that the frog is of a 

 wonderful pure blue. Sometimes the commoner large 

 edible frog is found with a similar absence of yellow pig- 

 ment (I found some in a garden near Geneva six years 

 ago), and then all the parts of its skin which usually are 

 green show as brilliant blue. 



It is at first difficult to believe that such fine, smoothly- 

 spread turquoise blue as that of the blue frog is due 

 merely to a " reflection effect," and that there is no blue 

 pigment present which would show as blue if light were 

 transmitted through it, or could be separated and 

 dissolved in some medium. Yet this is undoubtedly the 

 case. The nearest experimental production of such a 

 blue surface without blue pigment is obtained by first 

 varnishing a black board, and when the varnish is nearly 

 dry passing a sponge wetted with water over it. Some 

 of the varnish is precipitated from its solution in the spirit 

 (or it may be turpentine) as a fine cloud, and until the water 

 has evaporated it looks like blue paint, as the poet Goethe 

 found when cleaning a picture. It would be interesting 



