PERCEPTION. 515 



lip and the gums, while at the same time a 

 plate of zinc is laid on the tongue, or applied to 

 the inside of the cheeks ; and if a communica- 

 tion be then made between the two metals, 

 either by bringing them into direct contact, or 

 by means of a wire touching both of them at 

 the same time, a flash of light is seen by the 

 person who is the subject of the experiment. 

 This appearance is the effect of an impression 

 made either on the retina, or on the optic nerve, 

 and is analogous to that occasioned by a mecha- 

 nical 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 per- 

 ception 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 dis- 

 agreeable 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 



