THE SENSES 



117 



The eye is blind for all objects in the shaded region. By 

 resolving the various triangles the distance of the blind spot 

 from the central spot of the eye may be determined (3 to 

 4 mm.), and the diameter of the blind spot (1-5 mm.) may 

 also be ascertained. 



The shape of the blind spot may be mapped out by fixing 

 the head close to the paper, moving the point of the pencil 

 out till it disappears, and then moving it in different direc- 

 tions and marking when it re-appears. It is never quite 

 circular, and often shows rays extending from its edge which 

 are due to the blood-vessels. 

 (Experiment.) 



(2) The Field of Yision.- 

 The rest of the retina forward 

 to the ora serrata is capable 

 of stimulation, but when the 

 eye is directed forwards the 

 extent of retina stimulated 

 is influenced by the eyebrow 

 cutting off rays from above 

 and thus preventing the lower 

 part of the retina being 

 stimulated to its margin, 

 and by the nose intercepting 

 rays from the nasal side, thus 

 protecting the outer part of 

 the retina. 



The whole range of objects 

 which can be seen at one time 



constitutes the field of vision, and it, may be indicated by 

 the optical angle subtended by that range of objects. As 

 the distanc s e from the eye increases the field of vision expands. 

 It may be investigated by the perimeter, an instrument 

 which can readily be made by describing the arc of a circle 

 upon a blackboard or sheet of paper, placing the eye at 

 the centre and directing it to a mark in the middle of the 

 circumference, and then bringing a piece of chalk inwards 

 along the line until it is seen (Fig. 55). (Experiment) On 

 bringing an object from above it is not seen till A is reached, 

 while on bringing it from below it is seen at D. The angle 



FIG. 55. The Field of Vision, and the 

 method of investigating it by the 

 Perimeter. (7, the point in the arc 

 of a circle to which the eye B is 

 directed. 



