8S2 VISUAL SENSATIONS. [Book hi. 



changes in the retina which start the visual impulses, the passage 

 of these impulses along the optic fibres, and the changes in the 

 brain antecedent to consciousness beginning to be affected ; of 

 these the retinal changes probably take up the most time, but 

 into this point we cannot enter now. 



The length of the sensation, as compared with that' of the 

 stimulus, is illustrated by viewing objects in motion under a 

 very brief illumination, such as that of a single electric spark. 

 In such a case the light reflected from the object is sufficient to 

 generate a distinct sensation, to give rise to a distinct image of 

 the object, but it ceases before the object can make any appreci- 

 able change in its position, and the image accordingly is that 

 of a motionless object. When a moving body is illuminated by 

 several rapid flashes in succession, several distinct images cor- 

 responding to the positions of the body during the several flashes 

 are generated ; this, as we shall see presently, is because the 

 images of the body corresponding to the several flashes fall on 

 different parts of the retina. 



The duration of the stimulus remaining the same, the char- 

 acters of the sensation and the form of the sensation curve will, 

 in accordance with what was stated above, vary with the inten- 

 sity of the stimulus ; a bright flash will produce a sensation 

 greater and of longer duration than that produced by a feeble 

 flash, the curve will be higher and more extended. We have 

 reason to think, too, that the form of the curve is dependent on 

 the intensity of the stimulus in such a way that the decline from 

 the maximum begins earlier and at all events in the first part of 

 its course, is more rapid with the stronger than with the feebler 

 stimulus. 



When the stimulus is not a mere flash, but is of some dura- 

 tion leading to a prolonged sensation, we can readily distin- 

 guish between that part of the sensation which is going on 

 while the light is still falling into the eye, and that part which 

 goes on after the light has ceased to fall on the retina ; this 

 latter part is often spoken of as the after-image. When the 

 light is very bright this "after-image" frequently becomes very 

 prominent even after a very brief exposure. Thus, if we look, 

 even for a moment only, at the sun, and then immediately shut 

 the eye, an intense visual sensation, a bright visual image of the 

 sun, remains for some considerable time. After-images, espe- 

 cially as they are vanishing, are marked by certain features, 

 which we shall study later on, and which, as we shall see, are 

 related to the fatigue or exhaustion of the retina ; for the retina, 

 or rather the whole visual apparatus, is, we need hardly say, 

 subject to fatigue. 



Careful observation moreover has shewn that the visual sen- 

 sation curve is not a smooth one but broken in a remarkable 

 manner. When the retina is momentarily stimulated with a 



