CHAP, in.] SIGHT. 75 



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 sti- 

 mulus, 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 appreciable 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 corresponding 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 corre- 

 sponding to the several flashes fall on different parts of the retina. 



The duration of the stimulus remaining the same, the characters 

 of the sensation and the form of the sensation curve will, in accord- 

 ance with what was stated above, vary with the intensity 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 feeble stimulus. 



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

 leading to a prolonged sensation, we can readily distinguish 

 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, especially 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. 



749. From the prolonged duration of visual sensations it 

 results that when two or more stimuli, such as two or more flashes 

 of light, follow each other at a sufficiently short interval, the 

 two sensations or the several successive sensations are fused into 

 one more or less uniform sensation. Thus a luminous point moving 

 rapidly round in a circle gives rise to the sensation of a con- 

 tinuous circle of light. We might, in a very general manner, 



