chap, viii.j PIGMENTS. 271 



pigments, which are formed by protoplasmic cells, 

 or to the minute structure of the surfaces of parts 

 of their body, which variously affect the rays of 

 which white light is composed ; or to these two 

 causes combined. Although, in most cases, the 

 pigment is superficial, it is not always so ; thus, the 

 "colour" of a man's cheek is due to the haemo- 

 globin in the blood, as is shown by the yellow 

 colour of those affected with jaundice, in which 

 disease haemoglobin is converted into bile pigment ; 

 or the staining of the skin in syphilis, the poison of 

 which seems to be particularly destructive of the red 

 blood corpuscles. Similarly, the red eye of an albino 

 is due to the absence of pigment in the iris and the 

 retina, in consequence of which the red blood is 

 seen through the transparent tissues of the eye ; when 

 the retina is pigmented> but the iris free of pigment, 

 the red colour of the blood is, by interference, given 

 a blue shade, and the eye is said to be blue. When 

 pigment is laid down also in the iris, the red colour is 

 more or less completely obscured, and we get light- 

 brown or dark-brown eyes. 



No distinctive white pigment has yet been de- 

 tected, and the whiteness of certain animals must be 

 explained as caused by the presence of air-cells or 

 spaces in which none of the impinging light is ab- 

 sorbed. Many of the colours seen in animals are due 

 to the admixture of different pigments ; red overlaid 

 by yellow giving, for example, orange, or, when thinly 

 spread out, pink of various shades, proportionate to 

 the amount of colouring matter present. 



Many colouring matters are soluble in alcohol, and 

 not a few are fluorescent; in some cases they have 

 been observed to present absorption bands when 

 examined with the spectroscope, and these bands are 

 definite and characteristic of the pigment. Among 

 the Protozoa a blue colouring matter has been observed 



