THE SENSES 



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When the eye is fixed, and the visual field that is, the whole 

 space from which light can reach the retina in the given position, 

 ro, what comes to the same thing, the projection of the visual 

 field on the retina by straight lines passing through the nodal 

 point explored by means of a perimeter (Figs. 415, 416), it is 

 found that, under ordinary conditions, a white object is seen 

 over a wider field than any coloured object, a blue object over a 

 wider field than a red, and a red over a wider field than a green 



xn 



XI 



vm 



rv 



vn 



FIG. 416. PERIMETRIC CHART OF RIGHT EYE (AFTER HIRSCHBERG). 

 The numbers represent 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. 



object. The exact shape, as well as size, of the visual field also 

 differs somewhat for different colours. In disease of the retina, 

 or of the visual path between it and the cortex, or of the visual 

 cortex itself, the abridgment 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 monochromatic light of con- 

 siderable intensity can be perceived over the whole retina, yet 



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