PRACTICAL EXERCISES 1065 



(2) Keep the eye closed for a short time. Then direct it to a surface 

 illuminated by a weak blue light. A dark blue or almost black spot 

 (Maxwell's spot), corresponding to the macula, is seen in th? visual 

 field, owing to the absorption of the blue rays. 



Fig. 458. 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. 457. 



12. Ophthalmoscope (i) Human Eye (p. 991). Let A be the ob- 

 server, and B the person whose eye is to be examined. A and B 

 are seated facing each other. Suppose that the right eye of B is to 

 be examined. Close to the left ear of B is a lamp on a level with his 

 eyes : the room is otherwise dark. For a clinical examination, the pupil 

 should be dilated by putting into the eye a drop of a 0-5 per cent. 



