280 



CAPTAIN W. DE W. ABNEY ON THE COLOUR 



(XXI.) Curves shoiiring Equal Ordinates of Red, Green, and Blue Sensations to give 



While Light. 



The equation derived from the foregoing areas being taken as correct, when the 

 blue sensation replaces the violet, the final equation becomes 



BS OS BS W 



67-63 + 31-96 + -41 = 100, 



and we can find the curves which will have ordinates on such a scale that, when equal, 

 they give white. Taking the red curve as the standard, we must multiply the GS 



/>^./?o 



ordinates by - -, or 2'12, to make the areas of the two equal. When equal, the 



O-L't/O 



desired result is obtained as far as the green sensation is concerned. The BS ordi- 



/> 7.00 



nates must be multiplied by , or 165, to obtain the result for the third curve. 



These new ordinates are shown in Columns XVIII. and XIX., Table II., and these, 

 with Column XVII., are shown graphically in fig. 5. 



Fig. 5. 



'' 



?: 



X 



x 



N 



2 * 6 ft IO 12 H 10 Id 22(2* 2 28 30 52 J4|3e faa 4O *2 44 



\ 



46 40 SO 32 5* 59 5 



Sensation curves (prismatic spectrum) in which equal heights of ordinates form white. 



From these curves can be seen at a glance the positions that A and D, fig. 1, occupy, 

 and they show also those parts of the spectrum where the green and blue sensations 

 are seen, unmixed with any other sensation except white. These positions of A and D 

 we find to be about Scale Nos. 37 '3 and 3 5 '2, which are X5120 and 5060 respectively. 

 The purest green sensation is felt at the former Scale Number, and the purest blue 

 sensation is at Scale No. 23 '2, or close to the blue lithium line, which is at Scale 

 No. 22'8. As before said, the light of the crater is of that " whiteness " which closely 

 matches the white outside the colour fields ; hence it may be surmised that at these 

 two points we have the nearest approach to the true green and blue sensations. 



