52 SCIENCE FROM AN EASY CHAIR 



and compared it with that of the ordinary green tree-frog 

 in order to make out the cause of their difference in 

 colour. 



At Mentone there is a little shop where one may 

 purchase green tree-frogs and ornamental cages in which 

 to keep them. Every year the dealer has two or three 

 specimens of the blue variety on sale their backs and 

 heads looking like bits of turquoise-blue kid. Visitors 

 have sometimes wrongly supposed that the blue frogs 

 had been artificially changed in colour, but they are real, 

 natural varieties. A similar substitution of blue for green 

 has been noticed as a rare variation in other kinds of 

 frogs and toads in other countries. It really consists in 

 a suppression of yellow pigment. 



The interesting thing about the colour of the little 

 tree-frogs is that we find, on careful examination of the 

 skin of a dead specimen with the microscope, that there 

 is no green nor yet any blue "pigment" present in it 

 I found, on examining the blue specimen which died 

 after living three years with me, that there is only black 

 pigment overlaid by a colourless, semi-transparent layer 

 of skin. In this outer skin in the ordinary green speci- 

 mens there is scattered a quantity of excessively minute 

 yellow particles, which, mixed with the blue, produce the 

 green appearance. The fact is, that the wonderful " dead " 

 turquoise-blue of the blue frog is a colour-effect similar to 

 that of the blue sky and the blue of the human eye. It 

 is produced by a peculiar reflection of the light from 

 minute colourless particles, without the assistance of any 

 blue-coloured substance. The distinction of these two 

 modes of producing blue colour is important. 



Certain transparent bodies are so constituted that 

 when a beam of light is directed so as to pass through 

 them, the red, yellow, green, and purple rays which exist 

 in colourless sunlight are stopped, and only the blue 



