1064 



THE SENSES 



is covered. The arc is placed in a definite position, and the white 

 object gradually moved from the end of the arc until the person an- 

 nounces that he can just see it. The angle at which this occurs is read 

 off and recorded on the chart. The arc is then rotated into a new 

 position and the observation repeated. A line is drawn through all 

 the points thus obtained, and this constitutes the boundary of the 

 field of vision (Fig. 431). 



If the position of each point is inserted on the chart, a point above 

 the horizontal plane passing through the visual axis > being placed 



Fig. 455. 



Fig. 456. 



below it, and a point to the right of the vertical plane being moved to 

 the left, we obtain a map of the sensitive portion of the retina. Usually 

 perimeters are arranged to do this automatically. 



(2) Repeat the mapping of the field, using coloured papers (red, 

 green, and blue) instead of white. 



10. Mapping the Blind Spot. Make a black cross on a piece of white 

 paper attached to the wall, the centre of the cross being at the height 

 of the eye in the erect position. Stand about 12 inches from the wall, 



the chin supported on 

 a projecting piece of 

 wood. Fix the centre 

 of the cross with one 

 eye, the other being 

 closed, and move over 

 the paper a pencil 

 covered, except at the 

 point, with white 

 paper, until the point 

 j ust disappears . Make 

 a mark on the paper 

 at this point , and repeat 



Fig. 457- Map of Blind Spot (reduced by One-half). 

 Right eye. Distance of eye from paper 12 inches. 



the observation for all 

 diameters of the field. 

 The blind spot is thus 

 marked out (Fig. 457). 

 Its size and distance 



Its shape is not the same in all eyes (Fig. 458). 



from the fovea centralis can be calculated from the constructioiTgiven 



in Fig. 401 (p. 979). 



ii. The Macula Lutea, or Yellow Spot. (i) After closing the eyes 

 tor a minute or two, look with one eye through a strong solution of 

 chrome alum in a clear glass bottle with parallel sides. Hold the bottle 

 between the eye and a white screen or a white cloud. An oval rose- 

 co oured spot will be seen in a greenish field. The pigment of the 

 yellow spot absorbs the blue and green rays 



