SEC. 5. ON SOME GENERAL FEATURES OF VISUAL 



SENSATIONS. 



550. When light falls upon the retina it produces, under 

 favourable circumstances, a change in our consciousness which 

 we call a sensation of light, a visual sensation. The immediate 

 effect of the light is to stir up certain changes in the retina ; 

 these retinal changes give rise in turn to nervous changes in the 

 optic fibres ; these latter, which we have called 4 visual impulses,' 

 start in the brain a further series of events, one effect of which 

 is a change in our consciousness ; and it is this change in our 

 consciousness which we call a sensation. We may, and often 

 do, speak of light as a * stimulus' to the retina, the result of the 

 stimulation being visual impulses ; but we may also speak of 

 light as a stimulus to the whole visual apparatus, central as 

 well as retinal, regarding the sensation as if it were the direct 

 and immediate, instead of being the indirect and ultimate effect 

 of the stimulus. We may, by observing certain general features 

 of visual sensations, such as can be ascertained by means of a 

 direct and simple appeal to our own consciousness, study the 

 relations which obtain between the characters of the stimulus 

 on the one hand and those of the sensation on the other. There 

 are certain advantages indeed in doing this before we proceed 

 to discuss the nature of the changes in the retina through which 

 rays of light give rise to visual impulses in the optic fibres. But 

 in taking this course we must bear in mind how complex is the 

 whole process through which the stimulus gives rise to the 

 sensation. We must remember that, as we have already said, 

 though some of the characters of a visual sensation are impressed 

 upon it while it is as yet immature, as yet in the stage of visual 

 impulses, others are introduced later on in the course of the 

 cerebral changes. Since we are now dealing for the first time 

 with sensory impulses studied in this way, we may venture to 

 enter into some details, for the deductions which may be drawn 

 concerning visual sensations will apply to a large extent to other 

 sensations. 



To simplify matters we will in the first instance suppose that 

 the luminous object, the object emitting or reflecting light, is so 



878 



