654 THE SENSES 



When light falls on the rods and cones it produces changes which develop 

 nerve impulses that are transmitted by the chain of neurones extending 

 through the retina, the optic nerve and chiasma, the geniculate bodies, etc., 

 to the cerebral cortex of the occipital lobe, which is the sensorium for visual 

 sensations. We have already seen that the eye possesses a wonderful me- 

 chanical perfection for receiving and focussing light on definite parts of the 

 retina. A comparison of visual sensations shows that there are corresponding 

 qualities in the sensation, as, for example, its intensity, duration, localiza- 

 tion, complexity, etc. 



Duration of Visual Sensations. The duration of the sensation pro- 

 duced by a luminous impression on the retina is always greater than that 

 of the stimulus which produces it. However brief the luminous impression, 

 the effect on the retina always lasts for about one-twentieth of a second. 

 Thus, suppose an object in motion, say a horse, to be revealed on a dark 

 night by a flash of lightning, the image remaining on the retina during the 

 time of the flash. The object is really revealed for such an extremely short 

 period (a flash of lightning being almost instantaneous) that no appreciable 

 movement could have taken place in the period during which the stimulus 

 was produced on the retina of the observer. The horse would appear stand- 

 ing in the position of motion for about a twentieth of a second, though he 

 would not be seen to make any motions. And the same fact is proved in a 

 reverse way. The spokes of a rapidly revolving wheel are not seen as dis- 

 tinct objects, because at every point of the field of vision over which the re- 

 volving spokes pass, a given impression has not faded before another comes 

 to replace it. Thus every part of the interior of the wheel appears filled. 



The duration of the ajter-sensation produced by an object is greater in a 

 ratio proportionate to the duration of the impression which caused it. Hence, 

 the image of a bright object, as of the light of a window, may be perceived in 

 the retina for a brief period, the positive after-image. If, however, the primary 

 stimulation is sharp and intense there will follow presently an appearance of 

 the window in which all the contrasted lights are reversed, the negative after- 

 image. 



Intensity of Visual Sensations. It is quite evident that the more 

 luminous a body the more intense is the stimulus it produces. But the in- 

 tensity of the sensation is not directly proportional to the intensity of the 

 luminosity of the object. It is necessary for light to have a certain intensity 

 before it can excite the retina, but it is impossible to fix an arbitrary limit 

 to the power of excitability. As in other sensations, so also in visual sensa- 

 tions, a stimulus may be too feeble to produce a sensation. If it be increased 

 in amount sufficiently, it reaches a point that is intense enough to produce an 

 effect; this is a minimal or threshold stimulus. The amount of increase in 

 the stimulus that produces a perceptible change in the sensation is at 

 first very slight, but later quite great. It dees not depend on the absolute 



