930 PSYCHICAL FEATURES OF SENSATIONS. [Book hi. 



denly turning from one to the other. We are using ordinary 

 physiological methods when 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 pro- 

 ceeding from the tree give rise to the changes in the optic fibres 

 which we have called visual impulses. We have seen in deal- 

 ing with the brain reason to think (§ 478) 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 (§ 500) 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 



