AIM AND PKOGRESS OF PHYSICAL SCIENCE. 391 



senses, are, then, in no way dependent on the nature of 

 the exciting agent, but only on that of the nerve appa- 

 ratus under operation. 



The bearing of Miiller's law has been extended by 

 later research. It appears highly probable that even the 

 sensations of different colours and different pitch, as well 

 as qualitative peculiarities of luminous sensations inter se, 

 and of sonorous sensations inter se, also depend on the 

 excitation of systems of fibres, with distinct character 

 and endowed with different specific energy, of nerves 

 of sight and hearing respectively. The infinitely more 

 varied diversity of composite light is in this way refer- 

 able to sensations of only threefold heterogeneous 

 character, in other words, to mixtures of the three 

 primary colours. From this reduction in the number of 

 possible differences it follows that very different compo- 

 site light may appear the same. In this case it has been 

 shown that no kind of physical similarity whatever corre- 

 sponds to the subjective similarity of different composite 

 light of the same colour. By these and similar facts we are 

 led to the very important conclusion that our sensations 

 are, as regards their quality, only signs of external 

 objects, and in no sense images of any degree of re- 

 semblance. An image must, in certain respects, be 

 analogous to the original object ; a statue, for instance, 

 has the same corporeal form as the human being after 

 which it. is made ; a picture the same colour and per- 

 spective projection. For a sign it is sufficient that it 

 become apparent as often as the occurrence to be de- 

 picted makes its appearance, the conformity between 

 them being restricted to their presenting themselves 

 simultaneously ; and the correspondence existing between 

 our sensations and the objects producing them is pre- 

 cisely of this kind. They are signs which we have 

 learned to decipher, and a language given us with our 



