CH LI.] DISCRIMINATIVE SENSIBILITY 717 



tions, light, is easily measurable. Suppose a room lighted by 100 

 candles, and one candle more is brought in, the increase of light pro- 

 duced 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 1000 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 an 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 y^-. 



For weight it is T V to T V for different muscles. 



For tactile pressure -gV to T V 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 consciousness ; 

 they are illustrated by the sensations experienced during sleep 

 (dreams), and in the illusions to which mad and delirious people are 

 subject. 



Homologous stimuli. Each kind of peripheral end-organ is speci- 

 ally suited to respond to a certain kind of stimulus. The homo- 

 logous 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 

 instance, in the production of sensations of taste, touch, weight, and 

 alteration of temperature by conduction ; in the case of the olfactory 

 end-organs, the sensation is also excited by material particles given 

 off by the odoriferous body, and borne by the air to the nostrils. In 

 sound also, though there is no actual contact of the ear with the 

 vibrating body which emits the sound, the organ of hearing is excited 

 by waves of material substance, first of air, then of bones, then of 

 endolymph, and these excite the nerve-endings of the internal ear. 



