RESEARCHES BASED UPON THE THEORY 



241 



it is not necessary to take readings at all points of the curve. Readings 

 at SSN 48-8 (5770 A.U.), the point of intersection, and at SSN 42*8 

 (5410 A.U.), in the green, and SSN 53-2 (6090 A.U.), in the red, 

 usually suffice. 



We have now to consider some divergences from the calculated 

 curves. In most cases these are slight and are limited to the green- 

 blue, where differences may be expected, since the effect of the blue 

 sensation has been neglected in the calculated curves ; and further, 

 slight differences in macular pigmentation begin to be important in 

 this region. These divergences do not vitiate the method for purposes 



64 62 60 58 56 54 52 50 48 46 44 42 40 38 36 34 32 30 28 



Fig. 70. Luminosity curves of a normal trichromat with subnormal macular 

 pigmentation. + + + foveal ; O O O parafoveal. (Watson.) 



of diagnosis of protanopes and deuteranopes, since they occur in a 

 part of the spectrum where their characteristic colour defects are least 

 important from the practical point of view. 



Cases, however, occur of marked excess or deficiency of macular 

 pigmentation, and in these the luminosity curves show marked differ- 

 ences from the normal (Figs. 69, 70). Fig. 69 shows excess of pigmenta- 

 tion. Fig. 70 deficiency. By using a central fixation dot and a white 

 ring, the inner and outer edges of which subtend visual angles of 3° 22' 

 and 5°, Watson was able to obtain luminosity curves of a parafoveal 



p. c. V. 16 



