CH. L.] DISCRIMINATE SENSIBILITY. 693 



A definite example will help us to understand these mathe- 

 matical terms a little better. We will select our example from 

 the sense of vision, because the intensity of the cause of visual 

 sensations, light, is easily measurable. Suppose a room lighted 

 by 100 candles, and one candle more is brought in, the increase 

 of light produced by the extra candle is quite perceptible to the 

 eye ; or if a candle were removed, the decrease in light would be 

 perfectly appreciable. Next suppose the room lighted by 1,000 

 candles, and one extra was brought in, no difference would be 

 seen in the amount of illumination ; in order to notice increase or 

 decrease in the light it would be necessary to bring in ten extra 

 candles, or take away ten of the candles, as the case might be. 

 In each case an increment or decrease of one-hundredth of the 

 original light is necessary to cause a corresponding increase or 

 diminution in the sensation. 



This is after all a perfectly familiar fact ; a farthing rushlight 

 will increase the illumination in a dimly-lighted cellar, but it 

 makes no apparent difference in the bright sunshine. 



The magnitude of the fraction representing the increment of 

 stimulus necessary to produce an increase of sensation determines 

 what is called the discriminative sensibility. This fraction differs 

 considerably for different sense-organs ; thus : 



For light it is -j-^. 



For sound it is \. 



For weight it is T ' T . 



For temperature it is ^. 



For tactile pressure \ to in different parts of the body. 



Another general consideration in connection with sensation is 

 that the sensation lasts longer than the stimulus ; a familiar 

 instance of this is the sting after a blow. The after-sensations, as 

 they are called, have been specially studied in connection with the 

 eye (see After-images). 



Subjective sensations are those which are not produced by 

 stimuli in the external world, but arise in one's own inner con- 

 sciousness ; they are illustrated by the sensations experienced 

 during sleep (dreams), and in the illusions to which mad and 

 delirious people are subject. 



/fmologous stimuli. Each kind of peripheral end-organ is 

 specially suited to respond to a certain kind of stimulus. The 

 homologous stimuli of the organs of special sense may be divided 

 into : 



1. Vibrations set up at a distance without actual contact with 

 the object ; for instance, light and radiant heat. 



2. Changes produced by actual contact with the object ; for 



