124 PSYCHICAL FEATURES OF SENSATIONS. [BOOK in. 



we are studying how the various rays of light proceeding from a 

 tree form an image of the tree on the retina, and how these rays 

 thus falling on the retina give rise to visual impulses. But when 

 we study the change in our consciousness which is brought about 

 by the visual impulses thus excited through the image of the tree 

 falling on the retina, we are dealing with psychological problems. 

 The object, the tree itself, and our vision of it, the one being 

 commonly spoken of as the cause of the other, are connected by 

 a chain of events ; one end of the chain we study by physiological, 

 the other end by psychological methods ; and the difficulty of our 

 task arises from the fact that we have to use these two different 

 methods for a common purpose, namely that of explaining how 

 the tree gives rise to the vision of it. 



When we turn to the physiological side of the problem we 

 cannot at present say much more than that the rays of light 

 proceeding from the tree give rise to the changes in the optic 

 fibres which we have called visual impulses. We have seen in 

 dealing with the brain reason to think ( 643) that visual impulses, 

 like other sensory impulses, may influence the working of the 

 central nervous system without producing any such change of 

 consciousness as can be studied by psychological methods ; and we 

 further suggested ( 673) that in the structures of the mid-brain 

 which we called the primary visual centres a visual impulse 

 underwent a development by which it became no longer a mere 

 impulse but something more, and that the changes in these 

 primary visual centres transmitted to the occipital cortex gave 

 rise there to the changes with which the distinct affection of 

 consciousness is associated. It is undesirable to speak of the 

 events in the primary visual centres as "sensations," since it 

 is convenient to reserve this term for the psychical events, the 

 changes of consciousness of which we can become aware by 

 examining our own minds ; nor is there at present any need to 

 give them any name at all ; but it is important when we are 

 using the psychological method to remember that between the 

 physiological visual impulses and the psychological sensation there 

 are events which must not be ignored. 



Turning now to the psychological side of the problem we find 

 that the psychical events are also complex, and that the psychical 

 effects due to the same visual impulses are not all of the same 

 kind. This is seen even in the case of simple and isolated visual 

 sensations. Taking the effect of a luminous point, shining for a 

 moment only, as a simple form of visual sensation, we must 

 distinguish what we may call the mere change of consciousness, 

 the mere sensation of light, from the further psychical effect of 

 which we have already spoken and through which we associate 

 the sensation with a luminous point occupying a particular posi- 

 tion in external nature. Though the latter always accompanies 

 the former, though whenever we experience a visual sensation we 



