PRACTICAL EXERCISES 993 



field. The blind spot is thus marked out (Fig. 441). Its shape is 

 not the same in all eyes (Fig. 442). Its size and distance from the 

 fovea centralis can be calculated from the construction given in 

 Fig. 386 (p. 904). 



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



FIG. 442. COMPOSITE PICTURE OF BLIND SPOT (NOT REDUCED). 



The blind spot of the right eye was mapped ~by 31 men, the eye being always 

 at a distance of 12 inches from the paper. The maps were then superposed. 

 The amount of white at any point of the figure is intended to correspond to the 

 number of maps which overlapped at that point. Although the mechanical 

 process of reproduction gives rather an imperfect view of the composite map, 

 the area in the centre of the figure where the white is most continuous, and which 

 represents the shape of the majority of the blind spots, evidently bears a general 

 resemblance to the outline in Fig. 441. 



for 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-coloured spot will be seen in a greenish field. The pig- 

 ment of the yellow spot absorbs the blue and green rays. 



63 



