SECTION IV 



REGIONAL EFFECTS 

 CHAPTER I 



THE FIELD OF VISION FOR COLOURS 



When we look at or fixate a particular object we are conscious of 

 the presence and movement of other objects situated around the object 

 fixated. The image of the object fixated is formed at the fovea, those 

 of surrounding objects at various points in the peripheral parts of the 

 retina. The area over which such outlying objects can be perceived 

 is called the field of vision, which is therefore the projection outwards 

 of all the points upon the retina which can initiate visual sensations. 

 Its limits are usually plotted upon a chart by means of a perimeter^. 



The normal photopic field of vision for a well-illuminated white 

 object, 10 mm. square at a distance of 45 cm., is a horizontally oval 

 area extending upwards about 50°, outwards 90°, downwards 70°, and 

 inwards 60°. It varies with the illumination, the size of the test object, 

 the contrast of the test object with the background, and the state of 

 adaptation of the eye^. 



The fields for colours are smaller (Purkinje) : by the ordinary 

 clinical methods those for blue and yellow pigmented objects are about 

 10° smaller than that for white ; those for red and green 20° smaller, 

 the red field being usually smaller than the green, and violet smallest 

 of all. 



^ See Parsons, Diseases of the Eye, p. 1(50, 2iid ed. 1912. 



- See Landolt, in GraeJeSuemisch Hand. d. (jes. Augenheilkunde, iv. 1, 0-48, 1904 (witli 

 Bibliography). 



5—2 



