32 g NINETEENTH CENTURY. PT. III. 



reflection of the ray. By means of these methods it is 

 possible to measure very accurately how long a ray of light 

 takes in passing through any medium. Now, if Newton's 

 ' Corpuscular Theory ' had been true, a ray ought to pass 

 more rapidly through water than through air, but on the 

 Undulatory Theory the opposite should be the case. MM. 

 Fizeau and Foucalt have, however, found that light will 

 travel four miles in air in the time that it will only travel 

 three miles in water, thus giving almost demonstrative 

 proof that the undulatory theory of light is the true one. 



Colour Theory of Young and Helmholtz, 1801-1856. 

 We must now return to Dr. Young, for it was he who 

 first suggested the theory of the cause of colours being 

 distinguished by the eye, although this theory was neglected 

 until Professor Helmholtz worked it out more fully in our 

 day. Young suggested that there are three kinds of nerve- 



R O Y G JB V 



FIG. 600. 



fibre in the retina of our eyes, one of which is most strongly 

 affected by the long waves of light producing the sensation 

 called red, another affected by the medium waves producing 

 green, 1 and the third affected by the shortest waves producing 

 violet. Each of these sets of nerves would, however, be 

 slightly excited by the other waves as well, and so the 



1 Green, not yellow, as was formerly supposed. The study of 

 coloured light has shown that, so far as we can distinguish, there are 

 three primary colours ; these are red, green, and violet, for the ye/ low 

 of the spectrum is produced by a mixture of red and green. It is only 

 in paints or pigments that an effect of green is produced by the mixture 

 of blue and yellow. 



