THE LAND OF AZURE BLUE 55 



reflection from fine particles in the translucent, but turbid, 

 tissue of the iris overlying the dark, black chamber of the 

 eye. White geese and white cats frequently have blue 

 eyes, the blue being thus produced. The only pigment 

 which occurs in the human eye is a brown one, which 

 gives a colour varying from amber yellow to very dark 

 brown, almost black, according to the quantity present. 

 When a very little of it is present it gives, in combination 

 with the blue appearance of the unpigmented iris, a green 

 tint, so that green eyes owe their colour to the same 

 combination of causes as does the green skin of the little 

 tree-frogs, or " rainettes." 



No solvent will extract any pigment from the skin of 

 the blue frog nor by the finest trituration can one obtain 

 any coloured particles from it ; only fine black granules 

 can be separated. Alcohol removes the yellow pigment 

 from the skin of a green tree-frog (killed, of course, for the 

 experiment), and for a minute or two the skin becomes 

 blue when its yellow pigment is thus removed by immer- 

 sion in spirit ; but it rapidly becomes a dull greyish-brown 

 in colour, and so remains ; the green cannot be preserved 

 in spirit-specimens. It is not fully explained how such 

 a uniform " dead " blue is produced by the reflection of 

 light from fine particles, as that observed in the blue 

 frog's skin. 



It appears that the blue and the green colour in the 

 feathers of birds is in most, if not all, cases produced in 

 the same way as the blue and green of the tree-frog's skin. 

 It would be interesting were it found possible to produce 

 a full dead-blue colour by experimentally placing a coat 

 of a translucent but turbid colourless medium on a dead- 

 black plate. This, however, has not been done as a 

 deliberate experiment. It is, however, recorded that 

 Goethe was delighted to find what he considered to be a 

 confirmation of his theory of colour when a friend showed 



