CHAP, in.] SIGHT. 1271 



appears to have no objective cause at all. Moreover such 

 false sensations and perceptions having a distinctness which 

 gives them an apparent objective reality quite as striking 

 as that of ordinary visual perceptions, may occasionally be ex- 

 perienced not only when the eyes are closed, but even when the 

 eyes are open, and when therefore ordinary visual perceptions are 

 being generated, with which they mingle and with which they are 

 often confused. They are then spoken of as ocular phantoms or 

 hallucinations. They sometimes become so frequent and obtrusive 

 as to be distressing, and form an important element in some kinds 

 of delirium, such as delirium tremens. 



It is probable, as we have just suggested, that these false per- 

 ceptions may be started by events, which in ordinary language 

 may be called physiological; but the whole chain of events 

 between the visual impulse or even the immediate effect of the 

 impulse which we may consider as the physiological sensation, and 

 the terminal psychological perception is long and complex; the 

 discordance between the perception and its apparent cause, in 

 other words, the falsity of the perception, may be introduced in 

 the later, psychological, links of the chain. And an hallucination 

 may have such an origin that it may fitly be spoken of as purely 

 psychological. 



This naturally leads to the remark that a perception may be 

 revived in the mind, without the usual physiological antecedents, 

 as the result of purely psychological processes ; it is then generally 

 spoken of as an ' idea.' And we find, upon examination, that each 

 new perception which we experience is more or less modified 

 by memories and ideas resulting from bygone perceptions of a 

 like kind. But we have already determined to defer the con- 

 sideration of these and other more or less distinctly psychical 

 modifications of perceptions until we have studied certain results 

 arising from the use of two eyes. 



