XXXV.] THE SENSES. '9,5 



some degree of probability assign a physical ground for this 

 law. We have seen in a former chapter,^ that in nervous Probable 

 action something flows along the nerve-fibre with a mea- sensations 

 surable velocity much less than that of the electric current of differeut 

 along a wire. It is scarcely possible to doubt that what being 

 thus flows along the nerve, as well as what flows along the *'"?'?\^", , 



° ' * mitted by 



electric wire, is not a fluid, or matter in any form, but the same 

 energy, in the form probably of a vibratory or wave-like ^"it^o^t 

 motion of the molecules of the conductiu" fibre. Now combining, 

 when waves are formed in the same medium and of the 

 same order of magnitude, they combine into resultant 

 waves ; but if the waves formed in the same medium are 

 of different orders of magnitude, they do not combine, 

 but remain visibly distinct. Thus, if two stones of nearly 

 the same size are thrown into water at a little dis- 

 tance from each other, the waves raised by the two 

 will, where they meet, be seen to coalesce together, though 

 they afterwards separate ; but if the waves from a stone 

 are met by the waves proceeding from heavy drops of 

 rain, the two sets of waves, being of different orders of 

 magnitude, will remain visibly distinct, though intersecting 

 each other. If it is true that sensation is due to the 

 transmission of vibrations along the nerve-fibres to the 

 ganglia, it appears a probable conjecture that sensations of 

 the same sense are due to vibrations of the same order of 

 magnitude, and sensations of different senses to vibrations 

 of different orders of magnitude ; and that vibrations of the 

 same order, when coming into the sensory ganglia together, 

 combine and produce resultant sensations ; while those of 

 different orders do not so combine. But I only offer this as 

 a conjecture, wMch I see no possible means of verifying. 



2. The perception of space is due to the senses of touch Only 

 and sight exclusively : no otlier sense is capable of giving g-'.'i^^^^ve 

 it. I shall have to examine this subject in detail in the perception 

 next chapter, where I shall state reasons for believing 

 that our first cognition of space is due to touch, including 

 the muscular sense, though our habitual thoughts of space 

 have become associated rather with sight. 



' See the chapter on the Direction of Development (Chapter XII.). 



