RESEARCHES BASED UPON THE THEORY 247 



appear green to N., and he will never agree that the normal match 

 is correct. 



" If now we move the green slit to/. Fig. 74, where the normal green 

 sensation curve cuts the displaced curve we shall find that a match 

 which is correct for N. is also correct for R., and that either can detect 

 a small departure from this setting. 



" 3. The above is one arrangement of the three slits such that 

 R. and N. make the same match. Another such position is obtained 

 if the red slit is moved to g, Fig. 74 {SSN 52-4, 6000 A.U.). The light 

 which now conies through the red slit excites green sensation in the case 

 of both A^. and R., but to a greater extent in the case of the latter. 

 Since the red and blue sensations are the same for both it will be sufficient 

 to consider the equality of the red and green in the two cases. If the 

 width of the red slit is 0*62 times that of the green we get the following 

 values of the sensations on the equal area scale : 



where the sums are the same in the two cases and hence the mixture 

 appears white to both N. and R. Thus if the green slit is kept in a 

 constant position and the red slit is gradually moved up towards the 

 green the matches made by R. appear to N. at first too green, but the 

 excess of green gradually decreases till the red slit is at SSN 52-4. 

 If the red slit is moved further towards the green the mixture which 

 appears correct to R. will then appear too red to N. 



" 4. Owing to the displacement of the green sensation curve i^.'s 

 luminosity curve will be higher than the normal on the red side of the 

 point where the normal and displaced green sensation luminosity curves 

 cut and lower on the green side of this point, for the ordinates of the 

 luminosity curve are the sums of the ordinates of the three luminosity 

 sensation curves. The resulting luminosity curve for a displacement 

 of 2 SSN towards the red is shown in Fig. 73 by the dotted curve, the 

 corresponding normal curve being given by the thick continuous line." 



The observer (/?.), whom Abney and Watson examined, made a series 

 of matches throughout the spectrum, and from these his sensation 

 curves were deduced in the manner described on pp. 224, sqq. 



" His red and practically his blue sensation curves are identical with 



