THE ORGANS OF SPECIAL SENSE. 475 



cause and effect between the excitation of the retina under 

 the influence of light and the conscious perception of light 

 in the brain. Attempts have been made to establish a 

 mathematical relation between the two. That there is some 

 kind of a quantitative relation between the two is probable, for 

 we know that a stronger stimulation of the retina by stronger 

 light causes a stronger sensation. A harder stroke on the 

 piano causes an increased loudness in our mental percep- 

 tion. Two weights resting on the hand are perceived more 

 strongly than one. The question, however, is, what is, in 

 mathematical terms, this definite relation. Several attempts 

 have been made by means of extended experiments on the 

 various sense organs to determine such a mathematical 

 relation, the most noteworthy being that of the celebrated 

 psychologist Fechner and known as Fechner's " Psycho- 

 physical L,aw. ' ' This Psycho-physical law is, however, only 

 a modification of the psycho-physical law of the physiologist 

 Weber, whose work on the special senses is one of the 

 classics on that subject. This psycho-physical law says that 

 when the stimuli affecting the end organs vary in a geometric 

 ratio the intensity of the subjective sensation varies in an 

 arithmetical ratio. That is, if five units of light would pro- 

 duce a sensation one, then to produce a sensation twice as 

 strong requires twenty-five lights. To increase the subjec- 

 tive perception of the intensity of the light to three times 

 its original, would require one hundred and twenty-five 

 lights. Or, to state the law in another way, the subjec- 

 tive sensation increases directly as the logarithm of the 

 strength of the stimulus. 



While this law is applicable in many instances it is seri- 

 ously at fault in others. For instance, if one looks at a 

 line five inches in length and then wants a sensation of a 

 line twice as long it is of course nonsense to say that that 

 must be twenty-five inches in order to appear twice as long. 

 It approaches, however, the truth of things in connection 

 with the eye and ear. Every one knows that two candles 

 in a room do not make it twice as light as one. candle. 



