"So we have found that all the senses whose stimuli we are enabled to measure accurately, obey a uniform law. However various may be their several delicacies of discrimination, _this_ holds true of all, that _the increase of the stimulus necessary to produce an increase of the sensation bears a constant ratio to the total stimulus_. The figures which express this ratio in the several senses may be shown thus in tabular form: Sensation of light 1/100 Muscular sensation 1/17 Feeling of pressure, } " " warmth, } 1/3 " " sound, } "These figures are far from giving as accurate a measure as might be desired. But at least they are fit to convey a general notion of the relative discriminative susceptibility of the different senses.... The important law which gives in so simple a form the relation of the sensation to the stimulus that calls it forth was first discovered by the physiologist Ernst Heinrich Weber to obtain in special cases."[4] =Fechner's Law.=--Another way of expressing Weber's law is to say that to get equal positive additions to the sensation, one must make equal _relative_ additions to the stimulus. Professor Fechner of Leipzig founded upon Weber's law a theory of the numerical measurement of sensations, over which much metaphysical discussion has raged. Each just perceptible addition to the sensation, as we gradually let the stimulus increase, was supposed by him to be a _unit_ of sensation, and all these units were treated by him as equal, in spite of the fact that _equally perceptible_ increments need by no means appear _equally big_ when they once are perceived. The many pounds which form the just perceptible addition to a hundredweight feel bigger when added than the few ounces which form the just perceptible addition to a pound. Fechner ignored this fact. He considered that if _n_ distinct perceptible steps of increase might be passed through in gradually increasing a stimulus from the threshold-value till the intensity _s_ was felt, then the sensation of _s_ was composed of _n_ units, which were of the same value all along the line.[5] Sensations once represented by numbers, psychology may become, according to Fechner, an 'exact' science, susceptible of mathematical treatment. His general formula for getting at the number of units in any sensation is _S_ = _C_ log _R_, where _S_ stands for the sensation, _R_ for the stimulus numerically estimated, and _C_ for a constant that must be separately determined by experiment in each particular order of sensibility. The sensation is proportional to the logarithm of the stimulus; and the absolute values, in units, of any series of sensations might be got from the ordinates of the curve in Fig. 2, if it were a correctly drawn logarithmic curve, with the thresholds rightly plotted out from experiments.