VISION 



IOIQ 



different colours. In disease of the retina, or of the visual path 

 between it and the cortex, or of the visual cortex itself, the abridg- 

 ment of the field for white and for monochromatic light as mapped 

 out by observations with the perimeter is often of value in diagnosis. 

 Although it has been shown by Aubert and others that monochro- 

 matic light of considerable intensity can be perceived over the whole 

 retina, yet it may be said that the retinal rim is even then relatively 

 and, under ordinary conditions, absolutely colour-blind. This and 



XI 



X 



rv 



Fig- 431. Perimetric Chart of Right Eye (after Hirschberg). The numbers repre- 

 sent degrees of the visual field measured on the graduated arc of the perimeter. 

 w, boundary of field for white object ; b, for blue ; r , for red ; g, for green ; m, blind 

 spot ; M, medial, and L, lateral side of the field of vision. The Roman numbers 

 represent twelve meridians of the retina, each making an angle of 30 with 

 the next. They fix the ' longitude ' of any point in the field. The concentric 

 circles indicated by Arabic numbers represent angular distances from the 

 fixation point in the planes of these meridians. They give the ' latitude ' of any 

 point. 



other facts have given rise to thetheory (p. 1007) that the rods, which 

 are alone present at the ora serrata, are concerned in achromatic 

 vision (under conditions of dark adaptation), the cones in colour 

 vision as well as in achromatic vision (under daylight conditions). 



This brings us to the subject of colour-blindness, a phenomenon 

 of great interest in its theoretical as well as in its practical bearings. 



Colour-Blindness. A considerable number of persons (about 

 4 per cent, of all males, but only one-tenth of this proportion of 



