536 PSYCIIOLOGT. 



the greater the pre-existing stiinuhitioii is. Yvorn this in a general way 

 we can perceive the connection between the stimulus and tlie feeling it 

 excites. At least thus much appears, that the law of dependence is 

 not as simple a one as might have been expected beforehand. The 

 simplest relation would obviously be that the sensation should increase 

 in identically the same ratio as the stimulus, thus that if a stimulus of 

 strengtli one occasioned a sensation one, a stimulus of two should occa- 

 sion sensation two, stimulus three, sensation tfi7-ee, etc. But if this 

 simplest of all relations prevailed, a stimulus added to a pre-existing 

 strong stimulus ought to provolve as great an increase of feeling as if 

 it were added to a pre-existing weak stimulus ; the light of the stars 

 e.g., ought to make as great an addition to the daylight as it does to 

 the darkness of the nocturnal sky. This we know not to be the case : 

 the stars are invisible by day, the addition they make to our sensation 

 then is unnoticable, whereas the same addition to our feeling of the twi- 

 light is very considerable indeed. So it is clear that the strength of the 

 sensations does not increase in proportion to the amount of the stimuli, 

 but more slowly. And now comes the question, in what proportion 

 does the increase of the sensation grow less as the increase of the 

 stimulus grows greater. To answer this question, every-day experiences 

 do not suffice. We need exact measurements both of the amounts of 

 the various stimuli, and of the intensity of the sensations themselves. 



''How to execute these measurements, however, i.s something which 

 daily experience suggests. To measure the strength of sensations is, as 

 we saw, impossible ; we can only measure the difference of sen.sations. 

 Experience showed us what very unequal differences of sensation might 

 come from equal differences of outward stimulus. But all these ex- 

 periences expressed themselves in one kind of fact, that the same differ- 

 ence of stimulus could in one case be felt, and in another case not felt 

 at all— a pound felt if added to another pound, but not if added to a 

 hundred-weight. . . . We can quickest reach a result with our observa- 

 tions if we start with an arbitrary strength of stimulus, notice what 

 sensation it gives us, and then s\,e hoiv muchioe con increase the stim- 

 ulus without making the sensation seem to change. If we carry out 

 such observations with stimuli of varying absolute amounts, we shall be 

 forced to choose in an equally varying way the amounts of addition to 

 the stimulus which are capable of giving us a just barely perceptible 

 feeling of more. A light, to he just perceptible in the twilight need not 

 be near as bright as the starlight ; it must be far brighter to be just per- 

 ceived during the day. If now we institute such f)bservations for all 

 possible strengths of the various stimuli, and note for each strength 

 the amount of addition of the latter required to produce a barely per- 

 ceptible alteration of sensation, we shall have a series of figures in 

 which is immediately expressed the law according to which the sensa- 

 tion alters when the stimulation is increa.sed. ..." 



Observations according to this method are particularly 



