600 HANDBOOK OF PHYSIOLOGY. 



vealed on a dark night by a flash of lightning. The object would be seen 

 apparently for an eighth of a second, but it would not appear in motion; 

 because, although the image remained on the retina for this time, it was 

 really revealed for such an ^xtremely short period (a flash of lightning 

 being almost instantaneous) that no appreciable movement on the part 

 of the object could have taken place in the period during which it was 

 revealed to the retina of the observer. And the same fact is proved in a 

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

 distinct objects, because at every point of the field of vision over which 

 the revolving spokes pass, a given impression has not faded before an- 

 other comes to replace it. Thus every part of the interior of the wheel 

 appears occupied. 



The duration of the after-sensation, produced by an object, is greater 

 in a direct ratio with the duration of the impression which caused it. 

 Hence the image of a bright object, as the panes of a window through 

 which the light is shining, may be perceived in the retina for a consider- 

 able period, if we have previously kept our eyes fixed for some time on 

 it. But the image in this case is negative. If, however, after shutting 

 the eyes for some time, we open them and look at an object for an in- 

 stant, and again close them, the after image is positive. 



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

 luminous a body the more intense is the sensation it produces. But the 

 intensity 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 sensations, a stimulus may be too feeble to produce a sen- 

 sation. If it be increased in amount sufficiently it begins to produce an 

 effect which is increased on the increase of the stimulation; this increase 

 in the effect is not directly proportional to the increase in the excitation, 

 but according to Fechner's law, " as the logarithm of the stimulus/' 

 i. e., in each sensation, there is a constant ratio between the increase in 

 the stimulus and the increase in the sensation, this constant ratio for 

 each sensation expresses the least perceptible increase in the sensation 

 or minimal increment of excitation. 



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 distinctly felt. If, however, the first stimulus had been 

 that of an electric light, the addition of the light of a candle would 

 make no difference in the sensation. So, generally, for an additional 

 stimulus to be felt, it may be proportionately small if the original 

 stimulus have been small, and must be greater if the original stimulus 



