CHAP, in.] SIGHT. 



causes or other of a physiological nature give rise either in the 

 lower visual centres or in the cerebral cortex to just such 

 changes as would be induced by corresponding visual impulses, 

 though those impulses are wholly wanting; in other words the 

 causes in question give rise to visual sensations, in the physio- 

 logical meaning of that word, which produce a psychological 

 effect identical with that of visual sensations produced in the 

 ordinary way through the action of light on the retina. In 

 some cases perhaps the process may begin even in the retina 

 itself; abnormal changes in one or other of the retinal structures 

 may lead to the development of complex coordinate visual 

 impulses. 



Sometimes the sensations and perceptions thus occurring, 

 especially those which are met with on closing the eyes at night, 

 may be recognized as revivals, more or less altered, of sensations 

 experienced during the day; something sets going again the 

 series of cerebral events which were set going by actual rays 

 of light. These are generally spoken of as "recurrent sensa- 

 tions." 



At other times, there is no history of any like sensation 

 having been felt in the immediate past; the psychical effect 

 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 experienced 

 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 obtru- 

 sive 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 

 perceptions may be started by events, which in ordinary lan- 

 guage 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 sen- 

 sation, and the terminal psychological perception is long and 

 complex ; the discordance between the perception and its ap- 

 parent 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 antece- 

 dents, as the result of purely psychological processes; it is then 

 generally spoken of as an *idea.' And we lind, upon exam- 

 ination, that each new perception which we experience is more 



