HUMAN EYE COLOUR 43 



clear blue and grey eyes are of this class. The colour 

 of simplex eyes is not due to the colour of the fibrous 

 tissue of the iris itself, which consists of muscle 

 fibres, nerves, blood-vessels, and so forth, but to a 

 layer of dark purple pigment behind the iris, called 

 the uvea. In the new-born infant the fibrous tissue 

 of the iris is very delicate and transparent, so that 

 most of the colour of the uvea can be seen through 

 it : that is w^hy the eye of the new-born infant is 

 dark blue. A clear blue eye is due to the delicacy 

 of the fibrous tissue of the iris, which permits the 

 colour of the uvea to shine through. A pale blue or 

 grey eye is due to the coarseness or " stringiness " 

 of the fibrous tissue, which prevents most of the 

 colour of the uvea from being seen. Evidence that 

 the colour of very pale blue and of grey eyes is due 

 to the colour of the uvea which can filter through the 

 fibrous tissue in front of it, and not to the colour of 

 the fibrous tissue itself, is afforded by the fact that in 

 albinos in whom the uvea is colourless the iris 

 appears pink, this colour being given by the blood in 

 the fibrous tissue. The various colours of simplex eyes, 

 namely, the different intensities of blue and grey eyes 

 and the intermediates between these two colours, are 

 therefore due to differences in the texture of the 

 fibrous portion of the iris, which allow different 

 amounts of the colour of the purple uvea behind to 

 filter through. 



A simplex eye, therefore, is one in which there is 

 only one layer of pigment, the purple of the uvea: 

 a duplex eye is one in which there are two layers of 



