SECT. 7. ON SOME GENEBAL FEATUKES OF 

 VISUAL SENSATIONS. 



747. 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 ' 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 con- 

 sciousness 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 



