STUDIES IN SPECIAL SENSE PHYSIOLOGY .V'7 



Hence, within the region under examination, we may say with 

 fair accuracy, that the four subjects are grouped into a pair 

 relatively more sensitive to short-waved light and a pair relatively 

 more sensitive to long-waved light. The former are called prota- 

 nopes, the latter deuteranopes. 



But we have thus far only examined four cases, and must see 

 if the results can be substantiated with the help of a larger number 

 of subjects. If the conclusions just stated be valid, in matching 

 yellow some partial colour-blinds will require enormously more 

 red than others (protanopes) ; and this experiment, which can be 

 readily performed, will test our conclusions. V. Kries made the 

 test on twenty cases, using red (lithium line) and a fixed yellow 

 (sodium line). As will be seen from the table (p. 396) the subjects 

 do in fact fall into groups, one set (protanopes) requiring approxi- 

 mately five times as much red as the other class (deuteranopes) 

 to match the standard yellow. The classification is therefore a 

 tolerably definite one. 



This relative insensibility to long-waved light explains the 

 observation that in protanopes who correspond to the class badly 

 named " Red Blinds " the red end of the spectrum appears 

 shortened. The shortening is, however, of little interest from our 

 present standpoint. 



It is to be remarked that individual differences occur within 

 each class. In the two protatiopic cases, from 552 ^u/x onwards 

 to violet, one subject constantly demanded more red in his mixture 

 than the other. The subject requiring less red may be presumed 

 to have a deeply pigmented macula, so that the" homogeneous light 

 was weakened by selective absorption, because the diminution in 

 red values agrees with the known increments of absorption as we 

 pass towards the violet. The difference is of some importance in 

 connection with an interesting peculiarity of dichromatic vision. 

 Since daylight can be matched, for the dichromatic eye, by a 

 mixture of red and blue, and since all spectral colours can be 

 matched by mixing these standards, we should expect some spectral 

 colour to match more or less closely unanalysed daylight. The 

 situation of such a colour is called the " Neutral Point " of the 

 spectrum. As for protanopes the stimulus value of lights falls off 

 rapidly towards the red, their neutral point should be nearer the 

 violet than that of deuteranopes, and this is generally the case. 

 But, if there is a good deal of macular pigmentation, the mixed 



