vii RETINAL EXCITATION 339 



point of fixation is at the centre of the chart ; the concentric circles 10 apart 

 correspond to the perimeter scale ; the meridians are drawn at each 20. 



On comparing a number of visual fields mapped out carefully 

 from normal eyes there are found to be differences which may 

 amount to 15 or more in certain meridians. It is therefore 

 impossible absolutely to define " a normal field of vision." Landolt 

 gives the following figures for white : 



Above 70 



Above and outward 60 



Outwards ....... 90 



Below and outward ..... 85 



Below 60 



Below and inward 55 



Inwards ........ 55 



Above and inward . . . . . .55 



In making observations by means of the stars G. Ovio (1903) 

 found the extent of the visual field to be far wider than is usually 



FKJ. 164. Method to demonstrate Mariotte's blind spot, 



supposed about 90 in all directions, and even beyond 90 on the 

 temporal side. 



The more the object is illuminated the farther can it be seen 

 on the periphery ; large stars are therefore seen farther from the 

 fixation point than small stars. 



Another fundamental fact was discovered by Mariotte (1668). 

 He found in the region of the visual field which corresponds to 

 the position of the papilla of the optic nerve an area that is com- 

 pletely insensitive to light, and is therefore known as the blind 

 spot. 



To demonstrate its presence on one's self, it is only necessary 

 to close the left eye, and fixate the white cross of Fig. 164 with 

 the right, at a distance of 25-30 cm. On moving the book a little 

 nearer to or farther from the eye it is easy to find the exact dis- 

 tance at which the white disc to the right is totally invisible, so 

 that the black ground appears continuous. A black disc on a 

 white ground may be employed instead of a white disc on a black 



