DICHROMATIC VISION 171 



deuteranopes agree trichromats will also agree with them. The follow- 

 ing table (v. Kries) bears out this statement. 



Ratios of red (670 '8 MM) to yellow-green (550 MM) 

 in mixtures which match the homogeneous light are 



Wave-length of for deuteranopes for protanopes for trichromats 



homogeneous light too dark too light valid 



639 MM 0-012 0-026 0-016 



625 0-038 0-062 0-044 



613 0-07 0-12 0-09 



589 0-22 0-49 0-33 



569 1-00 3-00 1-34 



The stimulus values of the red and green can indeed be calculated 

 from the matches for the normal. The latter are given on p. 34 and 

 are repeated in the next table. We see that the normal requires 

 88-5 units of standard red to match, both in colour and brightness, 

 670-8 /z/t, and 71 units of standard green to match 550 /uju. A certain 

 deuteranope required 33 and 64 units respectively for the same matches. 

 Now for the normal trichromat, 



where Q A is the quantity of homogeneous light, Q r the quantity of red 

 (670-8 /J./JL), and Q g the quantity of green (552 yu.//) to make a perfect 

 match. 



But in the method used, i.e., with the spectrophotometer, Q A belongs 

 to one spectrum, and Q,. and Q (J belong to two spectra derived from 

 another source of light. By making r=\, we obtain the value of Q,- in 

 terms of the Q A spectrum, and similarly for g = \. 



Now, if observations under identical conditions by different observers 

 are directly comparable, 



Q r = 88'5 ; Q g = 71 ...... for the normal trichromat. 



Q r = 33 ; Q g = 64 ...... for the deuteranope. 



Therefore, in the intensity scale of the deuteranope 



& = Q' f + Q' 9 



where ^' and 



For example, the normal trichromat requires 202 units of red and 

 67 units of green to match 591 /A/A, or 



Q r>ai = 202Q, + 67Q,. 



We shall expect the intensity value of 591 ^ for the deuteranope to be 



202 x 33 67 x 64 



