THE PERCEPTION OF LIGHT AND COLOR. 195 



presents the peculiarity of being approximately proportional to the 

 absolute energy of the light, the intensity of which has been determined 

 iu the different parts of the spectrum by Laugley. From this propor- 

 tionality we may conclude that the light acts by itself in the visual 

 function, being without doubt integrally absorbed by a material which 

 is perhaps the pigment of Jacob's membrane — a pigment which sur- 

 rounds and separates from one another the rods and the cones. This 

 absorption of light would heat the pigment and therefore the visual 

 elements, and might generate at the same time thermo-electric currents. 

 Vibrations of an indeterminate nature should thus be produced in the 

 nerve fibers, and these vibrations should be similar to each other as to 

 form and wave length, whatever be the nature of the exciting rays, 

 since it is the pigment which directly produces the excitation; the 

 visual elements, therefore, can not of themselves provoke a color sensa- 

 tiou. It is quite easy to produce the luminous sensation independently 

 by means of any spectral ray whose intensity is too feeble to act on 

 the visual element, and, as is well known, a bluish-white sensation is 

 produced by all colors. On the other hand, the visual sensation may 

 also be isolated by fatiguing the eye by a white light, sufficiently 

 intense to decrease the excitability of the phototesthetic element 

 below that of the visual element, under which conditions even the red 

 does not appear colored. For the same reason all colors, if of sufficient 

 intensity, give rise to a white sensation, and hence the above conjectures 

 are found to be verified. 



This granted, the calorific action and the chemical action of light 

 both give rise to undulations in the nerve fibers, and it is probable 

 that their wave lengths differ, but it must be assumed that these 

 lengths bear a simple ratio to one another, since they must give rise 

 to a complex vibration of a definite nature to produce a definite 

 color sensation. Recalling that the relative amplitude of the two 

 kinds of vibrations varies with the different colors, that for a given 

 amplitude of visual vibration the amplitude of the photochemical 

 undulation increases rapidly from the red to the violet, if will be 

 recognized that the form of the undulation should vary with the ratio 

 of the two amplitudes— that is to say, with the color. 



Besides, M. Charpentier has demonstrated the existence of a reaction 

 time in the action of light on the organ of luminous sensibility — a 

 reaction time which increases from the red to the violet — while the 

 reaction time relative to visual sensibility does not vary with the 

 color. From this there results a new cause for variation in the form 

 of the resulting undulation, since the undulation of photochemical 

 origin does not coincide in phase with the undulation of pigmentary 

 origin. On the basis of this double difference of phase and relative 

 amplitude, and making certain arbitrary assumptions in regard to 

 the relative length of the two waves, as well as concerning the two 

 elements varying with the colors, M. Charpentier has constructed a 



