362 THE SENSORIAL FUNCTIONS. 



which produce them; for many are the instances in which 

 our actual perceptions are widely different from the truth, 

 and have no external prototype in nature. In the absence 

 of light, any mechanical pressure, suddenly applied to the 

 eye, excites, by its effect on the retina, the sensation of vivid 

 light. That this sensation is present in the mind we are 

 certain, because we are conscious of its existence: here there 

 can be no fallacy. But the perception of light, as a cause 

 of this sensation, bein^ inseparably associated with such sen- 

 sation, and wholly dependent on it, and corresponding in all 

 respects, both as to its duration and intensity, with the same 

 circumstances in the sensation, we cannot avoid having the 

 perception as well as the sensation of light: yet it is cer- 

 tain that no light has acted. The error, then, attaches to 

 the perception; and its source is to be traced to the mental 

 process by which perception is derived from sensation. 



Many other examples might be given of fallacious per- 

 ceptions, arising from impressions made in an unusual man- 

 ner on the nerves of the senses. One of the most remark- 

 able is the appearance of a flash of light from the transmis- 

 sion of the galvanic influence through the facial nerves. If 

 a piece of silver, or of gold, be passed as high as possible 

 between the upper 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 communication be then made be- 

 tween 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 mechanical 

 impulse, such as a blow directed to the same part of the ner- 

 vous 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, 



