346 THE SPECIAL SENSES. 



The end reaction of the activity of a sensory nerve is a state of con- 

 sciousness. The variations in magnitude of this state can not be 

 measured with objective exactness, they must be judged subjectively 

 by the individual concerned. A stimulus too weak to give a re- 

 sponse with a motor nerve is usually designated in physiology as 

 subminimal; a similar stimulus with sensory nerves is frequently 

 expressed by the equivalent term subliminal, that is, below the 

 threshold. So a stimulus just strong enough to provoke a percep- 

 tible reaction is the minimal stimulus for efferent nerves and the 

 threshold stimulus for sensory nerves. Inasmuch as the variations 

 in the intensity of consciousness can not be adequately measured, 

 it is customary, in studying the relations of the strength of stimulus 

 to the conscious response, to pay attention to the strength of stimu- 

 lus under any given condition which is sufficient to arouse a just 

 perceptible difference in the conscious reaction. Proceeding upon 

 this method, it is found in the case of the visual sensations and the 

 optic nerve, as with other sensations and their corresponding nerves, 

 that the increase of stimulus necessary to cause a just perceptible 

 change in consciousness varies with the amount of stimulus already 

 acting. If, for instance, the retina is being stimulated by a light of 

 1 candle power an increase of illumination to 1.1 candle power may 

 make a perceptible difference in sensation. But if the retina is 

 being illuminated by a light of 10 candle power an increase to 10.1 

 candle power would probably make no perceptible difference. For 

 a certain range of stimulation, in fact, it has been stated that the 

 increase in stimulus must be a constant fractional part of the stimu- 

 lus already acting. That is, in the hypothetical case given, if, with 

 1 candle power, an increase to 1.1 candle power makes a just per- 

 ceptible difference in consciousness, then with 10 candle power an 

 increase of -$ of the acting stimulus, namely 1 candle power will 

 be necessary to cause a perceptible difference. The relation as 

 expressed in this form is known as Weber's law ; but it seems prob- 

 able that, while the general fact is true, this exact expression of it 

 holds only approximately for an intermediate range of stimulation. 

 In this matter of a threshold stimulus the sensitiveness of the 

 retina shows also certain interesting differences in the foveal as 

 compared with the peripheral field. The difference is especially 

 marked when the reaction of the retina in strong lights is compared 

 with its reaction in dim lights. 



The Light-adapted and the Dark-adapted Eye. The con- 

 dition of the retina changes when after exposure to light it is sub- 

 mitted to darkness, the change being most marked in the peripheral 

 field. When one passes from daylight into a dark room vision 

 at first is very imperfect, but after some minutes it rapidly im- 

 proves, "as the eye becomes accustomed to the dark." The 



