458 THE SENSORIAL FUNCTIONS. 



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 letina, 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, being insepa- 

 rably associated with such sensation, 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 certain 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 fal- 

 lacious perceptions, arising from impressions made 

 in an unusual manner on the nerves of the senses. 

 One of the most remarkable is the appearance of a 

 flash of light from the transmission 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 com- 

 munication 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 efl'ect of an impression made either on the 

 retina, or on the optic nerve, and is analogous to 



