106 COLOUR VISION 



at once that it also holds good for the ordinary types of colour blind 

 (dichromats) 1 . 



The law has, however, been denied by Hering 2 and Tschermak 3 , 

 and Watson 4 has shown that it is at best an incomplete statement of 

 the facts. He confirmed the fact that colour matches remain valid 

 after previous stimulation with another light. He obtained the fol- 

 lowing results : 



Width of " green " slit 

 Resting Previously stimu- 



Character of match retina lated retina 



Correct 22'7 23'5 



Too little green 20'5 ll'O 



Too much green 25'0 27 '2 



In each case white was matched by a mixture of spectral red, green 

 and violet lights by altering the width of the slits through which the 

 coloured light proceeded. When the match was correct for the resting 

 eye it was found to be also correct for the eye which had been previously 

 stimulated with a colour, e.g., red. If now the slits through which the 

 red and violet lights passed were kept constant, while the green slit 

 was altered, being reduced until the deficiency of green was just observ- 

 able and opened until the excess of green was just perceptible, it was 

 found that the range through which the green slit could be altered 

 while still preserving a correct match with the white light was much 

 greater for the eye previously exposed to red light than for the eye 

 not thus previously stimulated. As shown above, the range between 

 a perceptible excess and a perceptible deficiency of green was 4'5 for 

 the resting eye and 16 - 2 for the previously stimulated eye. 



These experiments therefore show that the statement that " all 

 colour matches still remain valid," though true, is not the whole 

 truth, and is indeed misleading. After previous stimulation the range 

 of intensities which give a valid match is much wider than for the 

 resting eye. 



If we accept the law of the validity of optical equations irrespective 

 of previous stimulation, we can obtain a relation between the changes 

 in appearance of various lights caused by previous stimulation. If 

 R is the measure of the stimulus, the sensation will be aR, where a is 

 the measure of the retinal excitability for the particular stimulus R. 



1 v. Kries and Nagel. Zfsch. f. Psychol. u. Physiol. iL Sinnesorg. xu. 1, 1896 ; xxm. 

 161, 1900. 



2 Arch. f. d. ges. Physiol uv. 309 r 1893. 3 Ibid. LXX. 297, 1898. 

 4 Abney and Watson, Proc. Roy. Soc. Land. A. LXXXIX. 1913. 



