ON THE THEORY OF COMPOUND COLOURS. 433 



from the line joining (24) and (68) than in the other diagram, but I have not 

 obtained satisfactory observations of these extreme colours. It will be observed 

 that (32), (36), and (40) are placed further to the right in fig. 5 than in fig. 4, 

 shewing that the second observer (J.) sees more green in these colours than 

 the first (K.), also that (48), (52), (56), and (60) are much further up in fig. 5, 

 shewing that to the second observer they appear more blue and less green. 

 These differences were well seen in making an observation. When the instru- 

 ment was adjusted to suit the first observer (K.), then, if the selected colour 

 were (32), (36), or (40), the second (J.), on looking into the instrument, saw it 

 too green ; but if (48), (52), (56), or (60) were the selected colour, then, if right 

 to the first observer, it appeared too blue to the second. If the instrument 

 were adjusted to suit the second observer, then, in the first case, the other saw 

 red, and in the second green ; shewing that there was a real difference in the 

 eyes of these two individuals, producing constant and measurable differences in 

 the apparent colour of objects. 



XIV. Comparison by Curves of Intensity of the Primaries. 



Figs. 6 and 7, Plate VI. p. 444, are intended to indicate the intensities of 

 the three standard colours at different points of the spectrum. The curve marked 

 (R) indicates the intensity of the red or (24), (G) that of green or (44), and (B) 

 that of blue or (68). The curve marked (S) has its ordinates equal to the 

 sum of the ordinates of the other three curves. The intensities are found by 

 dividing every colour-equation by the coefficient of the colour on the left-hand 

 side. Fig. 6 represents the results of observations by K., and fig. 7 represents 

 those of J. It will be observed that the ordinates in fig. 7 are smaller between 

 (48) and (56) than in fig. 6. This indicates the feeble intensity of certain kinds 

 of light as seen by the eyes of J., which made it impossible to get observations 

 of the colour (52) at all without making the slit so wide as to include all 

 between (48) and (56). 



This blindness of my eyes to the parts of the spectrum between the fixed 

 lines E and F appears to be confined to the region surrounding the axis of 

 vision, as the field of view, when adjusted for my eyes looking directly at the 

 colour, is decidedly out of adjustment when I view it by indirect vision, turning 

 the axis of my eye towards some other point. The prism then appears greener 



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