INTENSITY OF VISUAL SENSATIONS 741 



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 after-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 pri- 

 mary stimulation is sharp and intense there will follow presently an appear- 

 ance 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 

 of 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 does not depend on the absolute change 

 of intensity of the stimulus, but is proportional to the intensity of the stimulus 

 already acting, Weber's law. 



This law, which is true only within certain limits, may be best under- 

 stood by an example. When the retina has been stimulated by the light of 

 one candle, the light of two candles will produce a difference in sensation 

 which can be easily and distinctly felt. If, however, the first stimulus is that 

 of an electric arc-light, the addition of the light of a candle will make no dif- 

 ference in the sensation. So, generally, for an additional stimulus to be felt, 

 it may be proportionately small if the original stimulus is small, and must 

 be greater if the original stimulus is great. The stimulus increases as the 

 numbers expressing its strength, while the sensation increases as the 

 logarithms. 



