PERCEPTION. 459 



that occasioned by a mechanical impulse, such as 

 a blow directed to the same part of the nervous 

 system, both being phenomena totally independent 

 of the presence of light. A similar fallacy occurs 

 in the perception of taste, which arises in the well- 

 known experiment of placing a piece of zinc and 

 another of silver, the one on the upper and the 

 other on the under surface of the tongue, and 

 making them communicate, when a pungent and 

 disagreeable metallic taste is instantly perceived : 

 this happens because the nerves of the tongue, 

 being acted upon by the galvanism thus excited, 

 communicate the same sensation as that which 

 would be occasioned by the actual application of 

 sapid bodies to that organ. Thus it appears that 

 causes which are very different in their nature, 

 may, by acting on the same nerves, produce the 

 very same sensation ; and it follows, therefore, that 

 our sensations cannot be depended upon as being 

 always exactly correspondent with the qualities of 

 the external agent which excites them. 



Evidence to the same effect may also be gathered 

 from the consideration of the narrowness of those 

 limits within which all our senses are restricted. 

 It requires a certain intensity in the agent, whether 

 it be light, or sound, or chemical substances ap- 

 plied to the senses of smell or taste, in order to 

 produce the very lowest degree of sensation. On 

 the other hand, when their intensity exceeds a 

 certain limit, the nature of the sensation changes, 

 and becomes one of pain. Of the sensations com- 

 monly referred to the sense of touch, there are 

 many which convey no perception of the cause 

 producing them. Thus a slighter impression than 



