A THEORY OF COLOUR VISION 385 



5. Let us now apply the results of the preceding section 

 to the eye. We have postulated vibrators in the eye to take 

 up the energy of the light wave ; we must also postulate a 

 nerve to carry the energy from the eye to the brain. The 

 preceding section shows that no matter how monochromatic 

 the incident light is, the motion of the vibrator will not be 

 monochromatic, but its energy will be distributed over a 

 range of wave-lengths ; for example, if the incident light is 

 yellow, the motion of the vibrator when analysed will contain 

 also some red and green. Consequently the energy passing 

 along the optic nerve to the brain will be distributed over the 

 red, yellow, and green with a maximum in the yellow. No 

 matter how monochromatic the incident light is, the impres- 

 sion on the brain is not monochromatic. 



The arrangement may be compared with a telephone. The 

 vibrators in the retina play the role of the diaphragm in the 

 transmitter which takes up the sound waves, and the optic 

 nerve plays the role of the telephone wire. There is, however, 

 one important difference. When a musical note is sung into 

 the telephone, it is transmitted correctly along the wire, because 

 the diaphragm reproduces the sound wave accurately. In the 

 eye the vibrators do not reproduce the wave accurately. It is 

 as if the diaphragm in the transmitter were subjected to dis- 

 turbing influences of such a nature that the person at the 

 other end hears a medley of musical notes over a range of 

 half an octave on each side of the note originally sung into 

 the transmitter. 



Any arrangement of molecular dimensions for taking up 

 the light wave must have some such modifying effect upon 

 its character. This fact has hitherto escaped notice, but must 

 receive attention as the modern views as to the nature of 

 white light become more prevalent. The sine curve of uniform 

 wave-length extending from negative to positive infinity was 

 an abstraction, and has given rise to erroneous ideas. 



Suppose, now, that the eye is simultaneously excited with 

 monochromatic red and monochromatic green light, say 

 lithium red and thallium green. Then two disturbances travel 

 along the optic nerve, one which has an energy distribution 

 given by the full curve on the right in fig. 7, and one which 

 has an energy distribution given by the full curve on the left. 

 If we add the ordinates of these two curves, we obtain the 



