132 OCULAR PHANTOMS. [BOOK m. 



Young-Helmholtz theory tends to lead us direct from the retinal 

 image to the psychological questioning of the sensations, and 

 seems to offer no bridge between the first step and the last, 

 Hering's theory is distinctly a physiological theory, and at least 

 holds out for us the promise of being able to push the physio- 

 logical explanation nearer and nearer home before we are obliged 

 to take refuge in the methods of psychology. 



784. We have seen ( 750) that visual sensations may be 

 produced in other ways than by light falling on the retina. In 

 such cases the effect which is produced upon our consciousness is 

 wholly misleading. A mechanical or electrical stimulation of the 

 retina may give rise to a visual sensation identical with that which 

 would be produced by the rays from a flash of light falling upon a 

 part of the retina. In both cases we should have a perception of 

 a flash of light occurring in a certain part of the field of vision ; 

 and so far as the perception itself is concerned we could not 

 distinguish between the latter which is a real and the former 

 which is a false perception. 



Not only single and simple sensations, but also complex groups 

 of sensations may be excited by other means than that of light 

 falling on the retina, and we may thus experience varied and 

 intricate perceptions which have no objective reality at all. Many 

 people when they close their eyes at night,- or indeed at other 

 times, see images of faces or other objects ; and though under 

 such circumstances it is easy to recognize the subjective origin of 

 the perception, that conclusion is reached by reasoning upon the 

 circumstances, and not because the perception itself differs in 

 character from a like perception caused by looking at an external 

 object. In such cases it is probable that some 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 physiological meaning of that word, which pro- 

 duce 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 sensations." 



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

 having been felt in the immediate past ; the psychical effect 



