348 



THE SPECIAL SENSES. 



been defined differently by various writers. We may consider it, 

 however, as the expression of the intensity of the acting stimulus. 

 Sensations of the same quality are easily compared as regards their 

 brightness. We can tell as between two whites or two greens which 

 is the brighter of the two, but when two different qualities a red 

 and a green sensation, for instance are compared our subjective 

 determination of the relative brightness is, for most persons, difficult 

 or impossible to make. To a lesser degree the difficulty is similar 

 to that of the comparison of sight and sound. According to the 

 conception adopted here, however, that the brightness is an ex- 

 pression of the intensity of the stimulus, an objective standard of 

 comparison might be obtained by measuring the resulting action cur- 

 rents in the optic nerve fibers. When the spectral colors are ex- 

 amined it is obvious that some of the colors are brighter than others, 

 the extreme red and extreme violet, for instance, possessing little 

 luminosity as compared v/ith the yellow. The relative brightness 



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Fig. 146. Diagram showing the distribution of the intensity of the spectrum as de- 

 pendent upon the degree of illumination. The spectrum is represented along the abscissa, 

 the numerals giving the wave lengths from red, 670, to violet, 430. The ordinates give 

 the luminosity of the different colors. Eight curves are given to show the changes in 

 distribution of relative brightness with changes in degree of illumination. With the 

 greatest illumination the maximum brightness is in the yellow (605-625) ; with weaker 

 illumination it shifts to the green (535) . (Kdnig. ) 



of the different spectral colors is found to vary with the amount of 

 illumination, as shown in the curves given in Fig. 146. With a 



