342 AIM AND PROGRESS OF PHYSICAL SCIENCE. 



of all our knowledge, a direct product of the vital principle, its 

 formative power would, in fact, then have attained the highest 

 degree of perfection. But an examination of the actual facts 

 at once destroys in the most merciless manner all belief in a 

 preordained harmony of the inner and external world. 



I need not call to mind the startling and unexpected results of 

 ophthalmometrical and optical research which have proved the 

 eye to be a by no means more perfect optical instrument than 

 those constructed by human hands; but, on the contrary, to 

 exhibit, in addition to the faults inseparable from any dioptric 

 instrument, others that in an artificial instrument we should 

 severely condemn ; nor need I remind you that the ear conveys 

 to us sounds from without in no wise in the ratio of their 

 actual intensity, but strangely resolves them and modifies them, 

 intensifying or weakening them in very different degrees, ac- 

 cording to their varieties of pitch. 



These anomalies, however, are as nothing compared with 

 those to be met with in examining the nature of the sensations 

 by which we become acquainted with the various properties of 

 the objects surrounding us. Here it can at once be proved 

 that no kind and no degree of similarity exists between the 

 quality of a sensation and the quality of the agent inducing it, 

 and portrayed by it. 



In its leading features this was demonstrated by Johannes 

 Miiller in his law of the Specific Action of the Senses. Accord- 

 ing to him, each nerve of sense possesses a peculiar kind of 

 sensation. A nerve, we know, can be rendered active by a vast 

 number of exciting agents, and the same agent may likewise 

 affect different organs of sense ; but, however it be brought 

 about, we never have in nerves of sight any other sensation 

 than that of light ; in the nerves of the ear any other than a 

 sensation of sound ; in short, in each individual nerve of sense 

 only that sensation which corresponds to its peculiar specific 

 action. The most marked differences in the qualities of sensation, 

 in other words, those between the sensations of different senses, 

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

 agent, but only on that of the nerve apparatus under operation. 



