VISION 119 



the right of the mark. Keeping the axis of vision fixed, 

 and the head at the same distance from the table, move 

 the pen slowly to the right. At a certain distance it will 

 become invisible; mark this spot on the paper. Carry 

 the pen still farther outwards. It will again become 

 visible; mark this spot also. The two spots will indicate 

 the outer and inner limits of the blind spot. Similarly 

 the upper and lower limit may be traced, and with a 

 * little practice an outline of the blind spot, showing even 

 the commencement of the retinal blood vessels as they 

 emerge from the edge of the optic disc, may be con- 

 structed. 



The size of the blind spot may be calculated from the 



formula ^ = ^, where/ is the distance of the eye from 



the paper, F the distance of the retina from the nodal 

 point of the eye (average = 15 mm.), d the diameter of the 

 outline on the paper, and D the outline of the blind spot. 



11. Region of distinct vision. Draw a circular dot 

 about 2 mm. in diameter, and round this draw eight 

 similar dots nearly but not quite touching it. Fix the 

 gaze on a mark on a piece of white paper about 30 cm. 

 distant. Close below this place the figure of nine dots, 

 and move it downwards, keeping it about 30 cm. from 

 the eye. At a very short distance the dots can no longer 

 be counted; then they become a blurred patch, and 

 probably a little farther they form a single mass. 



12. Purkinje's figures, a. Go into a dark room with 

 a lighted candle: looking steadfastly with one eye to- 

 wards a wall 1 ; hold the candle to the side of that eye so 



1 A light-coloured wall or white blind is the best. A wall, the paper 

 of which has any very marked pattern, should be avoided. 



