282 ELEMENTARY PHYSIOLOGY. [XXVII. 



commencement of the retinal blood-vessels as 

 they emerge from the edge of the optic disc, 

 may be constructed. 

 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 (aver- 

 age = 15 mm.), d the diameter of the outline on 

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



10. Region of Distinct Vision. 



Fix the axis of vision by steadfastly looking at 

 some mark. Make on a card two dots so close 

 that they can just be seen as two when placed 

 close to the mark. Keeping the axis fixed, move 

 the card towards the periphery of the field of 

 vision ; the dots will soon appear as one. 



Or, 



Make two dots on a card *5 mm. from each other. 

 Fix the axis of vision. Place the card outside 

 the whole field of vision and gradually move it 

 inwards towards the mark ; the dots will first 

 become visible as one, and only appear as two as 

 they approach the centre of the field, and enter 

 the region of distinct vision. 

 It will be found on examination that the outline 

 of this region is not circular but very irregular. 



11. Purkinje's Figures. 



Go into a dark room with a lighted candle : 



