514 THE SENSORIAL FUNCTIONS. 



correspondence either with the impressions them- 

 selves, or with the agencies 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 cer- 

 tain, because we are conscious of its existence : 

 here there can be no fallacy. But the percep- 

 tion of light, as a cause of this sensation, being 

 inseparably 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 falla- 

 cious perceptions, arising from impressions made 

 in an unusual manner on the nerves of the 

 senses. One of the most remarkable is the ap- 

 pearance 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 



