WEBER'S LAW. 931 



the sensation from the former is the more intense is an inference from knowledge 

 of laws connecting the sensation with the outward cause of sensation. We are 

 only at the beginning of such knowledge. We do know that a more intense 

 light applied to the excised eye causes a greater change in the electrical condition 

 of the optic nerve than does a similar but less intense light ; and that a heavier 

 tap on the web of the pithed frog causes a greater swing of the needle of a 

 galvanometer connected with the sensory nerve than does a lighter tap. Once 

 within the penetralia of the central nervous system, we have, however, still no 

 knowledge of the matter. A first step towards the problem would be to obtain 

 an exact measure for sensation itself. The stars invisible by day are brightly 

 perceptible by night. Yet they constitute to the untired morning eye an 

 external stimulus physically at least as potent as to the eye at night. One and 

 the same stimulus, according to the circumstances under which it operates, will 

 be perceptible either more or less or not at all. If to the stimulus of the star- 

 light there be added the overwhelmingly greater stimulus of daylight, the 

 sensations from the retinal points at which the stimulus is sunlight + starlight, 

 cannot be distinguished from the sensation generated by the rest of the retina 

 where the stimulus is daylight alone. As daylight fails, the brightest stars 

 become visible, followed by those less and less brilliant as the twilight 

 deepens. A stimulus to be felt may be so much the weaker, the weaker the 

 coexistent stimulation of the sensory organ, and must be so much the stronger 

 the stronger the coexistent stimulation. Evidently sensation does not alter in 

 identically the same progression as does stimulus. If a stimulus of intensity (1) 

 causes a sensation counted as 1, a stimulus of intensity (2) evidently does not 

 evoke a sensation 2, or stimulus (3) sensation 3, etc. Were it so, a sensible 

 stimulus added to a pre-existent strong stimulus would provoke as great a 

 difference (increase) of sensation as if added to a weak stimulus. Intensity of 

 sensation, therefore, increases less than directly proportionately with increase of 

 the intensity of the external stimulus. By noting for each strength of stimulus 

 the addition required to evoke a just perceptible alteration of sensation, a series 

 of quantities is obtained expressing the law according to Avhich sensation alters 

 when stimulation is increased. This expression is the so-called " law " of Weber. 

 It says that a given stimulus is perceived less when added to a large stimulus 

 than to a small one, or that an addition to a large stimulus is perceived less than 

 an addition to a small one, unless it, relatively to the stimulus, is as great. 

 The " law " may be phrased variously in physiological theory. It is true that 

 it may be interpreted as not physiological at all, but psychological. The dis- 

 proportion between increment of stimulus and increment of sensation may take 

 place in purely psychological events and processes. Wundt is of that opinion. 

 He points to the wide occurrence of such a ratio in all psychical activity 

 as outcome of the relativity inherent in every conscious process. Waller 

 finds the response in a nerve trunk directly stimulated, as judged by 

 action current, increase much more nearly directly as the increase of 

 external stimulus than does the response from muscle when nerve is stimu- 

 lated, or from nerve when retina is adequately stimulated. 1 Waller's evidence 

 seems to point to the law being in part a function of nerve cell endings ; 

 probably, therefore, applicable to synapses as to motor plates. Delboeuf con- 

 siders the law an expression of an ever-increasing proportion of loss of effect in 

 the central nervous system, due to "fatigue." The notion of fatigue has been 

 in the physiology of the senses expanded so as to include phenomena to which 

 the term is perhaps little suitable, e.g. Helmholtz's theory of successive contrast. 

 Delboeuf 's seems similarly a strained use of the term. A process need not be 

 of the nature of fatigue to make an ever larger part of the work of the stimulus 



1 Brain, London, 1895, vol. xviii. ; see also Fick, " Untersuch. u. elekt. Reizung," 

 Braunschweig, 1869 ; Hermann, " Handbuch d. Physiol.," Bd. i. Ahth. 1, S. 107 ; J. J. 

 Midler, Arb, a. d. Zilrichc Hochschuh, 1869 ; Dewar and M'Kendrick, Nature, London, 

 July 1873; F. C. Muller, Arch. f. Physiol., Leipzig, 1886. 



