168 COLOUR VISION 



20 parts of blue. Hence 6 or 10 parts of blue will have relatively- 

 little stimulus value when mixed with 30 to 35 parts of red, whilst a 

 trace of red will manifest itself by definitely increasing the brightness 

 when mixed with 60 parts of blue. 



The red curves show distinctly the difference in the two groups 

 of dichromats. No blue is necessary for the match until beyond 530 ju/x. 

 The red protanopic maximum is at 571 jUju, ; the sharp fall towards 

 the red end shows the relatively low stimulus value for the long-wave 

 light. The deuteranopic maximum is at 603 jm/x, and the curve does 

 not fall to so low a value at the red end. Rivers^ has therefore intro- 

 duced the terms scoterythrous and photerythrous for protanopes and 

 deuteranopes respectively. The advantage of these terms is that they 

 are purely descriptive of observed facts and are quite independent of 

 any theory. Unfortunately they have not been generally adopted, 

 and it has been thought best in this book to use v, Kries' terminology. 

 The protanopes are relatively more sensitive to the short-wave light, 

 the deuteranopes to the long-wave. In order that a yellow and a red 

 may look equally bright a protanope will require a much more intense 

 red than a deuteranope. This fact cannot be explained on physical 

 grounds, such as macular pigmentation, since these rays are not 

 appreciably absorbed by the yellow pigment. 



Further to prove that the fact is true of the two groups v. Kries 

 made thirteen equality of brightness observations on 9 deuteranopes 

 and eleven on 11 protanopes, using the red lithium line (670-5 /z/x) with 

 the yellow sodium (D) line (589-2 /x/x). The protanopes required the 

 following quantities of red : 214, 213, 211, 205, 196, 198, 210, 200, 210, 

 203, 235 : the deuteranopes required : 36-5, 36-3, 36-3, 36-5, 38-4, 37-3, 

 37-0, 37-0, 37-8, 37-0, 36-9, 38, 40. The protanopes therefore require 

 on an average five times as much red as the deuteranopes in order 

 that the red may appear as bright as the standard yellow. These 

 observations confirm earlier results obtained by Bonders^. 



The low stimulus value of red for the protanopes accounts for the 

 so-called " shortening of the red end of the spectrum " in this class of 

 dichromats. The limit of visibility of the red end, however, varies 

 with normal people, and depends upon the intensity of the light, 

 whether the whole spectrum or only the isolated colour is observed, 

 and so on. Too much stress has often been put upon this point. 



The blue curves show a general agreement, but more individual 



1 Schafer's Text Book of Physiologij, 1091, 1900. 



2 Arch.f. A licit, u. Physiol. 528, 1884. 



