226 COLOUR VISION 



GS, we shall see later, give the sensation of yellow. If we place a slit 

 in the violet and move another slit along the less refrangible part of 

 the spectrum, we shall find a place where this colour and violet together 

 make a white, the slits being opened or closed to make the match. This 

 position, then, is that in which the red and green sensations are equally 

 stimulated, and answers to A. In B we have a green and violet with 

 equal ordinates and a deficiency of red. If we place a slit in the red 

 and move another about in the green, we shall find a colour which with 

 the red makes white. This position, then, will have an equal stimulation 

 of green and violet. This gives another fixed point. The next point to 

 determine is shown diagrammatically by C, which illustrates the green 

 we have to look for, mixed only with white. This is more difficult to 

 find, as it would require a purple to be added to make a match with the 

 white, and this does not exist in the spectrum. Suppose we mix A with 

 B, we get a diagram of the kind shown in the fourth diagram. There 

 are equal reds and violets stimulated, but a larger stimulation of green 

 sensation. This gives a colour paler than the spectrum colour, but 

 still a green which can be matched. There are also other plans, 

 dependent on trial and error, for fixing this point. 



When the observations for obtaining the fixed points have been 

 made it will be found that the complementary of the violet is at scale 

 number (SSN) 48-7 (577-2 ^/x), that of the red at SSN 34-6 (500 /*/*), 

 and that where green is mixed only with white is at SSN 37-5 (515 /x/x). 

 These are therefore the points of intersection of the three sensation 

 curves (Fig. 64). 



It is next necessary to ascertain the amount of white in the green 

 at SSN 37-5. One slit is placed in this situation and another at SSN 

 59-8, the position of the red lithium line. The luminosities of these 

 colours, with equally wide slits, are taken by comparison with another 

 light, such as yellow or white. They are found to be 39-2 and 9-4 

 respectively. A patch of yellow light from a second spectrum, derived 

 from the same source of light, is placed beside the mixture of red and 

 green. In order to obtain a colour match it is necessary to add white 

 to the homogeneous yellow. From the slit widths and the luminosity 

 of the added white an equation in terms of luminosity is obtained : 



a (yellow) + b (white) = c (red) + d (green) 

 or a (yellow) = c (red) + d (green) b (white). 



Since the red contains no white the percentage of white in the green 



