RESEARCHES BASED UPON THE THEORY 249 



position of the red slit is altered. Another advantage of the white 

 match is that the yellow produced by mixing green and red to match 

 the D light is less saturated than the D light itself, and this causes 

 considerable difficulty with some observers when making the match. 



" When matching white by a mixture of violet light (SSN 9-5, 

 4235 A.U.) and yellow light the following results were obtained : 



W.W. 

 Position of yellow slit . . . . SSN 48'9 or 5780 A.U 



R. 

 SSN 50-0 or 5860 A.U. 



showing that the complementary to the violet is in the case of R. dis- 

 placed towards the red, as has been shown on p. 246 we should obtain 

 if the green sensation curve was shifted towards the red end of the 

 spectrum. 



' Again it was found that if the red and violet slits were in the 

 standard positions d and b, Fig. 74, a position for the green slit, /, 

 E'ig. 74, could be found such that the mixed light matched white both 

 for R. and for N. 



" The luminosity curve obtained by R., using the equality of bright- 

 ness method, agrees with what it ought to be if his green sensation curve 

 is displaced towards the red by about 2 SSN, such a calculated curve 

 being given in Fig. 73. Attention may be drawn to the fact that R.'s 

 luminosity curve does not agree with the normal at SSN 48-6, as it 

 would if R.'s abnormality were due to a deficiency of either the red or 

 green sensations " (v. p. 238). 



' It is of practical importance to consider what effect a shift of the 

 green sensation curve such as that exhibited by R. will have on the 

 power of discriminating colours, particularly those colours which are 

 used as signals at sea and on railways. As has been mentioned, one 

 effect of the displacement of the green sensation curve is that the part 

 of the spectrum which to the normal appears yellow, to such persons 

 appears green or greenish. Thus R. places the change from green to 

 yellow in the spectrum at SSN 49-9 or 5810 A.U. while to a normal 

 (W.W.) this point appeared to be at SSN 48-8 or 5780 A.U. One effect 

 of this difference is that a light, such as that given by a paraffin lamp, 

 which to the normal appears decidedly yellow appears to a person with 

 the shift of a greenish hue and in fact R. often calls such a light green. 



' Another effect of the displacement is that the perception of a green 

 light when diluted with white light is very much more difficult than for 



