Theory of Colour Vision. 113 



To make the agreement with fig. 2 perfect it is necessary to 

 make k 2 vary from colour to colour, and this variation was 

 determined in the following manner: — AN, the tangent 

 to the vertex of the experimental curve was drawn, and 

 perpendiculars were then let fall to AN from each of the 

 sixteen points ; thus the perpendicular from P was PN. 

 The distance of the foot of the perpendicular from A in 

 arbitrary units was taken as s, values on the violet side being 

 taken negative, and values on the red side positive. The 

 parabola represented by the broken line was next drawn to 

 pass approximately through the sixteen points. It cuts PN 

 in Q. PQ was then measured in the diagram, and expressed 

 in the same units as NQ. The results are shown in the 

 following Table : — 



X. 



6559 



6302 



6063 



5842 



Table III 



s. 



232 



217 

 190 

 150 

 124 



PQ. 



90 



98 

 94 

 73 



38 



P. 



215 

 223 

 219 



198 



5638 -. 



163 



5451 



5281 



5128 



110 

 10-3 



8-0 



15 



3 



-24 



-14 



- 1 



140 

 128 

 101 



4991 



4869 



61 

 1-7 



111 



124 



4759 



4659 



4569 



... - 7-1 

 ... -127 

 ... -18-8 

 ... -21-1 

 ... -234 



-13 



36 



25 



118 



179 



112 

 161 

 150 



4488 



4413 



243 

 304 



4342 



... -24-4 



247 



372 



PQ represents P except for an additive constant. The 

 latter must, of course, be sufficiently great to remove the 

 negative sign. I have taken 125 for its value ; if the 

 energy-curves have roughly the shape of probability curves, 

 and red and blue are added, this value allows the resultant 

 curve a double maximum, but at the same time ensures that 

 there is only a simple maximum when the component curves 

 are closer together in the spectrum than red and blue. 



It will be obvious now to a mathematician that any 

 problem in colour-mixing can be solved by means of the "s 

 and k 2 columns in the above Table, and the result will be 



Phil Mag. S. 6. Vol. 38. No. 225. Sept 1919. 2 F 



