FARADAY-TYNDALL PHENOMENON 71 



producing the gorgeous sunset colours (see Spectroscope, Chap. II.). 

 In one of Tyndall's experimental verifications of this theory he 

 passed light through a tube containing a mixture of gases (butyl 

 nitrate in air and hydrochloric acid in air), which gradually 

 combined to form a dust-like suspended precipitate. At first the 

 particles were exceedingly small and the colour seen from the side 

 of the tube was a delicate tint of blue. As the particles increased 

 in size the blue became more intense, " until at length a whitish 

 tinge mingled with the pure azure, announcing that the particles 

 were now no longer of that infinitesimal size, which scatters only 

 the shortest waves." 



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 sufficient 

 depth appears blue due to 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 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 finely 

 suspended unpigmented colloid particles 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. Except with people who have 

 very black eyes, the pigment on the posterior surface of the iris 

 does not develop at birth. That is, most babies are bom with 

 deep blue eyes. As they become older the colloidal particles 

 become larger and the blue becomes less intense. Further, the 

 uveal pigment may develop and the colour change from blue to 

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

 (Bancroft). 



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

 of non-absorbed light. Complete absorption of light gives rise 

 to the sensation of black. A perfect reflecting surface would be, 

 of course, invisible. If follows that particles of different sizes 



