THE CONCEPTUAL WORLD 15 



vibration of the fork, or of the air in its vicinity, we 

 say that pitch differences are quantitative ones, and 

 that the states of consciousness which accompany these 

 physical events are also quantitatively different. 



So also with colour. If we had no such apparatus 

 as prisms or diffraction gratings, which enable us to 

 find what is the wave length of light, should we have 

 any idea of the spectral hues, red, yellow, orange, green, 

 etc., as differing from each other quantitatively? It 

 is certain that we should not. But observation and 

 experiment have shown that the nerve-endings of the 

 optic nerve in the retina are stimulated by vibrations 

 of something which we agree to call the ether of space, 

 and that the frequency of vibration of light which we 

 call red is less than that which we call orange, while 

 the frequency of vibration of orange light is less again 

 than that of blue light, and so on. To our conscious- 

 ness red, orange, yellow, and blue light are absolutely 

 different, but we disregard this intuition and we say 

 that our perceptions of light are similar in kind but 

 differ, in some of them are more intense than are 

 some others. Again, have we any intuitive knowledge 

 of increasing temperature ? If we dip our hands into 

 ice-cold water the sensation is one of pain, if the water 

 has a temperature of 5° C. it feels cold, if it is at 15° C. 

 we have no particular appreciation of temperature, if 

 at 25° C. it feels very warm, if it is at 60° it is very hot, 

 and if it is at 90° we are probably scalded and the 

 feeling is again one of pain. If we place a thermometer 

 in the water we notice that each sensation in turn is 

 associated with a progressive lengthening of the 

 mercury thread, and if we investigate the physical 

 condition of the water we find that at each stage the 

 velocity of movement of the molecules was greater 

 than that at the preceding stage. We say, then, that 



