248 COLOUR VISION 



those of the normal, but his green sensation curve is markedly different. 

 It is similar in shape to the normal, but is displaced by about 2 SSN 

 towards the red end of the spectrum. Thus to him the maximum 

 for the electric arc light occurs at wave-length 5690 A.U. in place of 

 at 5575 A.U., which is that of the normal. 



" When white is matched by mixing light which passes through 

 three slits placed at the points d, e, and 6, Fig. 74 (6705, 5190, 



o 



4235 A.U.), R. requires very much more green than the normal, as is 

 shown by the first line in the next table. If, however, the red slit is 

 moved towards the yellow, the green and violet slits remaining fixed in 

 position, the excess of green required by R. got less and less, till finally 



Changes in slit-widths required to match white by R. and a normal trichi'omat (W.W.). 

 The positions of the green and the violet slits were kept constant, as was also the 

 width of the green slit, the match being obta.ined by varying the widths of the red 

 and violet slits and the brightness of the comparison white. With the red slit at 

 52 '5 the match made by either was correct for the other. 



a position for the red slit was found where the mixture matched white 

 both to R. and to the normal. If the red slit is moved further towards 

 the yellow R. required less green than the normal, so that his mixture 

 looked slightly red to the normal. The above changes indicated that 

 in the case of R. we had to deal with a shift of the green sensation 



curve. !: 



" When matching D light with a mixture of red and green light, 

 if the red is at the lithium line, R. required considerably more green 

 than did the normal. If, however, the red slit were moved towards 

 the yellow, just as in the case of the white matches, the excess of green 

 gradually decreased, though, owing to the fact that the D light in the 

 case of R. excites the green sensation more strongly than the normal, 

 we did not get the marked change in the appearance of R.'s match to 

 the normal which has been referred to in the case of the white match ' 

 (v. p. 247). " For this reason the white match is preferable to the 

 Rayleigh match for bringing out the characteristic changes when the 



