DISCRIMINATION AND COMPARISON. 535 



of strength one; a stimulus of strength tivo begets a sensation of 

 strength tivo., or three., or four, etc. But to do this we must first know 

 what a sensation two, three, or four times greater than another, 

 signifies. . . . 



" Space magnitudes we soon learn to determine exacth', because we 

 only measure one space against another. The measure of mental mag- 

 nitudes is far more difficult. . . . But the problem of measuring the 

 magnitude of sensations is the first step in the bold enterprise of mak- 

 ing mental magnitudes altogether subject to exact measurement. . . . 

 Were our whole knowledge limited to the fact that the sensation rises 

 when the stimulus rises, and falls when the latter falls, much would not 

 be gained. But even immediate unaided observation teaches us certain 

 facts which, at least in a general way, suggest the law according to 

 which the sensations vary with their outward cause. 



" Every one knows that in the stilly night we hear things unnoticed 

 in the noise of day. The gentle ticking of the clock, the air circulating 

 through the chimney, the cracking of the chairs in the room, and a 

 thousand other slight noises, impress themselves upon our ear. It is 

 equally well known that in the confused hubbtib of the streets, or the 

 clamor of a railway, we may lose not only what our neighbor says to us, 

 but even not hear the sound of our own voice. The stars which are 

 brightest at night are invisible by day ; and although we see the moon 

 then, she is far paler than at night. Everyone who has had to deal 

 with weights knows that if to a pound in the hand a second pound be 

 added, the difference is immediately felt ; whilst if it be added to a 

 hundredweight, we are not aware of the difference at all. . . . 



" The sound of the clock, the light of the stars, the pressure of the 

 pound, these are all stivmli to our senses, and stimuli whose outward 

 amount remains the same. What then do these experiences teach ? 

 Evidently noticing but this, that one and the same stimulus, according 

 to the circumstances under which it operates, will be felt either more or 

 less intensely, or not felt at all. Of what sort now is the alteration in 

 the circumstances, upon which this alteration in the feeling may depend ? 

 On considering the matter closely we see that it is everywhere of one 

 and the same kind. The tick of the clock is a feeble stimulus for our 

 auditory nerve, which we hear plainly when it is alone, but not when it 

 is added to the strong stimulus of the carriage-wheels and other noises 

 of the day. The light of the stars is a stimulus to the eye. But if the 

 stimulation which this light exerts be added to the strong stimulus of 

 daylight, we feel nothing of it, although we feel it distinctly when it 

 unites itself with the feebler stimulation of the twilight. The pound- 

 weight is a stimulus to our skin, which we feel when it joins itself to a 

 preceding stimulus of equal strength, but which vanishes when it is 

 combined with a stimulus a thousand times greater in amount. 



"We may therefore lay it down as a general rule that a stimulus, 

 in order to be felt, may be so much the .smaller if the already pre-exist- 

 ing stimulation of the organ is small, but must be so much the larger. 



