AIM AND PROGRESS OF PHYSICAL SCIENCE. 343 



The bearing of M tiller's law has been extended by later re- 

 search. It appears highly probable that even the sensations of 

 different colours and different pitch, as well as qualitative pecu- 

 liarities of luminous sensations inter se, and of sonorous sensa- 

 tions 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 infi- 

 nitely more varied diversity of composite light is in this way 

 referable 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 composite light may appear the same. In 

 this case it has been shown that no kind of physical similarity 

 whatever corresponds 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 resemblance. 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 depicted makes its ap- 

 peai-ance, the conformity between them being restricted to their 

 presenting themselves simultaneously ; and the correspondence 

 existing between our sensations and the objects producing them 

 is precisely of this kind. They are signs which we have learned 

 to decipher, and a language given us with our organisation by 

 which external objects discourse to us a language, however, 

 like our mother tongue, that we can only learn by practice and 

 experience. 



Moreover, what has been said holds good not only for the 

 qualitative differences of sensations, but also, in any case, for 

 the greatest and most important part, if not the whole, of our 

 various perceptions of extension in space. In their bearings on 

 this question the new doctrine of binocular vision and the in- 

 vention of the stereoscope have been of importance. All that 



