RESEARCHES BASED UPON THE THEORY 233 



If the green component is absent the visible spectrum will be the 

 same length as to the trichromat. From SSN 65 to 50, and from 

 SSN 16 to the end of the spectrum he will have isolated pure sensations, 

 "red" and "blue" respectively. At SSN 37-5 (515 /^^la), where the 

 two equal-area curves intersect, he will see " white " : this is his neutral 

 point, and we see that it is rather nearer the violet end than in the 

 protanope. 



In some parts of the spectrum it is easier for the dichromat to measure 

 the luminosities of spectral lights than for the trichromat. In fact near 

 the neutral points it is a simple matter of homo- as opposed to hetero- 

 chromatic photometry. Theoretically, the protanopic plus the deuter- 

 anopic luminosity curves should equal the trichromatic plus one blue 

 luminosity curve. The extra blue curve makes very little difference, 

 as it is relatively very small compared with the other two {v. Fig. 63). 

 Moreover, its effect is further diminished by macular absorption. As 

 already mentioned, we have no means of making absolute measure- 

 ments, but the relative values hold good. 



We should expect the discrimination sensibility for hues in the 

 spectrum to be very defective in dichromats. The first experiments 

 of this nature were made by Brodhun^, himself a deuteranope. His 

 curve is given in Fig. 4. We see that he has only one maximum, 

 at 500 /x/Li, near the F line, but here the curve is very sharp and the 

 value of SA is very low, so that his discrimination sensibility is very 

 great at this point. 



Steindler^ examined three protanopes and five deuteranopes (Figs. 

 66, 67). She found that SA was much larger than for the normal in 

 all cases. The deuteranopes showed only one maximum, the average 

 being at 503 [xfi. The protanopes showed two maxima, at 500 /x/a and 

 598 /x/x. On the same basis of calculation as for trichromats {v. p. 230) 

 the maxima for deuteranopes should be at 500 /a/x and 635 /x/x. 

 The first position agrees well with that found experimentally ; at 

 635 jjifi the luminosity was too low for deuteranopes for accurate obser- 

 vations to be made. The calculated maxima for protanopes were 

 500jLt)M and 600 /Xju, which agree excellently with the observations. 

 Tritanopes are so rare that they could not be examined ; their theoreti- 

 cal maxima are at 540 /x/x and at 620 /x/x. 



Konig^ calculated that the number of hues which a deuteranope 

 could discriminate in the spectrum was 140, as compared with the normal 



1 Ztsch. f. Psuchol. u. Physiol, d. Sinnesorg. in. 89, 1892. 



2 Sitz. d. Wiener Akad. cxv. ii a. 115, 1906. 3 Konig, p. 368. 



