62 MR. J. SMITH ON THE ORIGIN OF COLOUR 



or rather two different colours, a blue and a purple, I now 

 began to experiment on the effects of different velocities. 

 I found that when I reduced the rate of motion the blue 

 often became purple, but I also found that the same effect 

 was produced when the white ray was carried into the 

 shade, or properly speaking by reducing the intensity of the 

 white ray with which I was operating. In the shade I could 

 not by a more rapid motion raise the purple up to blue, for 

 no rapidity of m.otiou could raise the value of the incident 

 light or the light reflected from the paper; although by 

 reducing the pulsations of the incident light, or the light 

 falling on the paper even in the shade, I could reduce its 

 value. 



93. This is the same phenomenon as that of the card 

 painted partly black. In a faint light the black appears 

 through the card as purple; in a stronger light as blue. 

 The one experiment illustrates the other, and both demon- 

 strate the pulsatory nature of light. Of course light must 

 proceed by waves, but there is no proof of length of wave, 

 in the common acceptation of the term according to the wave 

 hypothesis, having any effect in the generation of colour. 

 Nor has there been in this experiment any decomposition 

 of light. The same ray has been used, namely, the white ray 

 reflected from the paper. The frequency of the vibrations 

 has only been increased or diminished. Still it must be 

 understood that the mere increase or diminution of the 

 number of vibrations will not produce colour, or every pe- 

 numbra would be coloured. But every penumbra is no 

 more coloured than every refracted ray is coloured. Every 

 known substance on which light falls, or through which it 

 is transmitted, casts a shadow, but no shadow can be 

 coloured unless a ray at an angle to tlie ray which forms 

 the shadow is deflected upon that shadow, as was explained 

 in the candle experiment. Hence the use of refraction. 

 It resolves multiple force ; it does not separate quick rays 



