COLOUR OF DISPERSIONS 77 



The colour of some samples of stained glass is caused not by an 

 even distribution of the pigment or stain throughout the glass, but 

 by the dispersion of fine metallic particles. Water of sullicient 

 depth appears blue l)ecause of the presence of tiny suspended 

 particles. If larger particles are present, some light of longer 

 wave-length, e.g. yellow, is diffracted and the colour becomes green. 

 The water of the Rhone as it leaves Lake Geneva is intensely blue, 

 while the Rhine at Strassburg is green. The Rhine contains about 

 70 per cent, more calcium carbonate in suspension than the 

 Rhone. 



Tyndall observed that the blue of the eye has a similar origin 

 to the blue of the sky, the sea, and the Rhone, viz. scattering of 

 light from small suspended particles. The uvea, the dark pig- 

 mented double layer at the back of the iris, prevents the reflection 

 of light and prevents the colour of the blood in the vessels behind 

 it from becoming apparent. In an albino this pigment is absent 

 and the eye appears pink. The colour of blue eyes is due to fine 

 unpigmented colloid particles suspended in the iris. The various 

 colour stages between the blue and the grey eye arise from differ- 

 ences in the mean size of the dispersoid particles — the finer the 

 particles, the more intense the blue. In brown and black eyes, 

 pigment cells are found in the endothelium in front of the iris. 

 Except with people who have very black eyes, the pigment on the 

 anterior surface of the iris does not develop at birth. That is, most 

 babies are born with deep blue eyes. As they become older the 

 colloidal particles become larger and the blue becomes less intense. 

 Further, if the pigment develops the colour changes from blue to 

 hazel, brown or black. The reverse change never takes place 

 (Bancroft). 



Colour may be due, as we saw in Chap. II., to the reflection 

 of non-absorbed light. A surface which completely absorbed light 

 would give rise to the sensation of black, while a perfect reflecting 

 surface would be, of course, invisible. It follows that particles of 

 different sizes will " select " light of certain wave-length for 

 absorption, and, as a consequence, colour may result from " selec- 

 tive " absorption, reflection or diffraction. 



In the table on p. 78, from Ostwald, is given the relationship 

 between size of particle and colour (a) from light absorbed, and 

 [b) from light transmitted (Table XI.). 



One must, however, take into account the other optical com- 

 ponents, e.g., refractive index of medium. The absorbed colour 

 given below does not necessarily indicate the colour of light 

 scattered by the particles. 



As the particle becomes smaller, the colour transmitted alters to 



