THE CONCEPTUAL WORLD 13 



than between smells and sounds. Different musical 

 sounds are more like each other than are sounds, and 

 feelings of heat and cold. There is a greater likeness 

 between the states of consciousness which arise from 

 the stimulation of the same receptor organ, than 

 between those that arise from the stimulation of 

 different receptors. Those differences of sensation 

 accompanying the stimulation of different sense organs 

 we regard as different in kind ; there is absolutely 

 no resemblance between a colour and a sound, we 

 say, however much the modern annotator of concert 

 programmes may suggest the analogy. But we say 

 that there may be different degrees of stimulation of 

 the same sense organ, and that the sensations that we 

 thus receive are of the same kind though they differ 

 in intensity. The whistle of a railway engine becomes 

 louder as the train approaches, that is to say, more 

 intense, and if we study the physical conditions that 

 are concomitant with the stimulation of our tympanic 

 membranes we shall see that waves of alternate rare- 

 faction and compression are set up in the atmosphere 

 outside our ears. All the time that the train approaches 

 the frequency of these waves remains the same, that 

 is, just as many occur in a second when the train is 

 distant as when it is near. But the amplitude of the 

 waves has been increasing, and the velocity with 

 which the molecules of air strike against the tympanic 

 membranes becomes greater the nearer is the source 

 of sound. We can represent this by means of a 

 diagram which shows that the amplitude of the waves — 

 which represents the loudness of the sound — increases 

 while the frequency — which represents the pitch — 

 remains the same. The amplitude is represented by 

 the straight vertical lines, u, 22, 33, etc., which are 

 of increasing magnitude. Thus we represent the 



