RESEARCHES BASED UPON THE THEORY 245 



since the multiplier is twice as great as for the normal, the resulting 

 curve will be the same as for the normal. 



" Now, if we take a yellow at SSN 48-4 (5760 A.U.), i.e. at a (Fig. 74), 

 and mix it with a violet at SSN 9-5 (4235 A.U.), i.e. at b (Fig. 74), 

 the width of the violet slit being 2-5 times that of the yellow, and read 

 off the three sensations at these places from the full line curves given in 

 Fig. 74, the values in the violet being multiplied by 2-5, we get the 

 following numbers : 



Position Sensations 



of slit Red Green Bine 



a. 67-9 69-3 1-fi 



b 1-4 67'7 



Sums 69-3 69'3 69-3 



Since the sums for the three sensations are the same it follows that 

 the mixture will look white to the normal eye. Further, since the 

 curves for the person who has half the green sensation are precisely 

 similar, the sums will be equal for him also and hence he will match 

 the mixed colour with his own white. Similarly for any other case of 

 colour defect, where the defect is due to a deficiency of one of the 

 sensations, the curves of the sensations on the equal area scale will be 

 the same as those of the normal eye. 



' Although the person who has the defect of one of the sensations 

 will agree with the normal match, it will be found that when making 

 the match the position of the yellow slit can be moved some little 

 distance from the correct position for the normal without the match 

 becoming defective to him." Watson has measured the range over 

 which the match is valid by making himself artificially colour-blind by 

 fatiguing his eye with a colour (v. p. 106). 



' If the sensation curves are the same for a given observer as for 

 the normal, except that one of them is shifted along the spectrum, 

 quite a different result will be obtained. Thus suppose that the green 

 sensation curve is shifted towards the red end of the spectrum by an 

 amount equal to 2 SSN and occupies the position shown by the dotted 

 curves in Figs. 73 and 74. The sensations at the points a and b for 

 such a person are as follows : 



Position Sensations 



of slit Red Green Blue 



a 67-9 79-2 1-6 



b 1-4 67-7 



Sums 69-3 79'2 09'3 



