RELATION OF SENSATION TO STIMULUS 535 



aware of the meaning attached to such qualities or such sensations 

 as sweet, red, hard, high-pitched (of sound), &c. It would be abso- 

 lutely impossible to compare these sensations among themselves. 

 We cannot say, for instance, that this sound is louder than that 

 colour is red. Such fundamental differing qualities of sense are 

 spoken of as the modality, of the sensation. 



On the other hand, within the sensation evoked by any one^sense- 

 organ we find differences of quality which are more comparable 

 among themselves. Thus we can compare the pitch of various 

 sounds, or the colour of various objects seen with the eye. We 

 can even say that the bitter taste of any substance is more marked 

 than the sweet taste of another. The question arises how far these 

 differences in sensation correspond to and are a measure o differences 

 in the physical events by which they have been evoked. A very little 

 consideration suffices to show that there is no resemblance between 

 a sensation and the stimulus, and that one and the same physical 

 event applied to different sense-organs will evoke absolutely distinct 

 sensations ; while different modes of stimuli applied to one sense- 

 organ will always evoke the same sensation. Thus if light falls 

 on the retina it causes a sensation of light. If the same radiant 

 energy, consisting of transverse vibrations in the ether, be allowed 

 to fall on the skin, it either produces no sensation at all, or, if con- 

 centrated by means of a burning-glass, may give rise to a sensation 

 of warmth, heat, or pain. If we take a tuning-fork which is vibrating 

 100 times per second and apply it to the surface of the skin, we get 

 simply a sensation of vibration, i.e. a series of tactile impressions 

 repeated at rapid intervals. If the same tuning-fork be applied to 

 the head, its vibrations are imparted to the bones of the skull and 

 thence to the auditory nerve-endings and arouse in our consciousness 

 a tone-sensation of a certain note. The same thing happens if the 

 vibrations of the tuning-fork are conducted by the ear to the auditory 

 nerve- endings in the ordinary way through the external and middle 

 ear. On the other hand, a sensation of light may be aroused not only 

 by the incidence of radiant energy of a certain wave length on the 

 retina, but also by electrical or mechanical stimulation of the retina. 

 If the eye be turned inwards and the finger be pressed on the eye 

 through the outer canthus of the lids, a sensation of light is aroused 

 and we see a coloured circle which we refer to some spot lying to the 

 nasal side of the eye stimulated. The character of the sensation bears 

 therefore no resemblance to the physical events by which the sensation 

 is evoked, but depends entirely on the nature of the sense-organ which 

 is stimulated. A sensation of light may be produced by stimulation 

 in any way of the retina, or of the optic nerve, or of the terminations of 

 the optic nerve in the brain. In the same way stimulation of an 



