764 THE PHYSIOLOGY OF CONSCIOUS STATES [CH. LII. 



Fechner's interpretation of Weber's law is, however, open to serious 

 criticism, into which we cannot enter here. 



Weber's law is but an expression of everyday experience. A 

 rushlight will brighten a dark cellar, but its presence is unfelt in 

 sunshine. So, too, if a room be lighted by 100 candles, and if one 

 candle more be brought in, the increased illumination produced by 

 the extra candle would be just perceptible to the eye. But if a 

 room were lighted by 1000 candles, no appreciable difference would 

 result from the introduction of an extra candle. Ten candles would 

 have to be introduced, in order to effect a just noticeable difference. 

 In each case a difference of one-hundredth of the original strength 

 of stimulus is needful to cause a just appreciable difference in the 

 sensation; and this is in accordance with Weber's law. 



For light, the fraction is about y^ ; for noise, it is about J ; for 

 cutaneous pressure, it varies between ^ and ^ ; for weight, between 

 T V and ^V, according to the part of the body which is under 

 investigation. 



A sensation requires an appreciable time for its development. 

 Part of this time is spent at the end-organ on which the stimulus 

 acts, part in conveying the nervous impulse along the sensory nerve 

 to the brain, and part within the brain itself. This latent period 

 varies in length according to the sensation ; e.g., it is longer for sight 

 than for sound, and longer for pain than for touch. 



A sensation outlasts its stimulus. Indeed, a single stimulus 

 may produce a whole train of after-sensations. These are specially 

 noticeable in the case of visual sensations, which we shall be 

 considering later. 



When the sensation and its after-sensations have passed away, 

 the original experience may still be revived, either spontaneously or 

 by an effort of volition. This revival involves what is called the 

 memory image. When, in this way, a tune " comes into the head," 

 we recognise that it is only a reproduction, or a representation, of 

 what we have previously heard. 



Occasionally, however, the revived image has all the vividness 

 and distinctness of objective experience, and we believe that it is 

 "real." In other words, we have a hallucination. Hallucinations 

 occur normally in all people ; but they are, of course, particularly 

 common in sleep and in conditions of insanity or delirium. 



It is still disputed whether the difference between original and 

 revived experiences corresponds to an excitement of distinct regions 

 of the brain. Some physiologists have gone so far as to speak of 

 " memory centres " as existing apart from the sensory centres which 

 are supposed originally to have excited them, and they have 

 considered that the recall of a scene or of a tune is due to the re- 

 excitation of the appropriate memory centres, while the correspond- 



