THE SKIN 



^93 



the granules may remain unchanged. It is quite well estab- 

 lished that the green is a combination effect of light reflected 

 from the guanin granules, and the golden pigment through 

 which the light passes. As the light reflected from the 

 granules contains a large proportion of blue rays, we have 

 what is practically equivalent to a blue background seen 

 through a yellow medium, the result of which is to produce 



Fig. 51. — Sections through the skin of the tree frog. In A the skin 

 appears yellow; the black pigment is concentrated, and considerable 

 light is reflected through the yellow chromatophores from the deeper 

 tissues. In B the color of the skin is green ; the black chromatophores 

 are in a state of moderate extension, forming a dark layer beneath the 

 yellow cells, so that most of the light passing through the yellow cells is 

 reflected from the bluish granules. ' In C the pigment from the black 

 chromatophores has surrounded the yellow cells, giving the skin a very 

 dark color. (From Gaupp, after Ehrmann.) 



green. The yellow medium absorbs most of the colors of 

 the spectrum, allowing yellow and a certain amount of green 

 Hght to pass through. The blue background reflects only 

 blue and green light. Since green rays are the only ones 

 which are capable both of reflection from the blue back- 

 ground and of passing through the yellow medium, the back- 

 ground appears of a green color. 



