PLATE VIL. 
Projected images of the blind spot, B, and of the yellow spot, Y, as 
seen by three observers. The fine lines correspond to actual 
retinal distances of half a millimetre ; hence the squares represent 
retinal areas of half-millimetres squared. 
Nothing is easier than an ocular proof of the blind spot. Close the 
right eye, and look steadily at the cross on this page with the left eye. 
When the eye is about 12 or 13 inches from the paper, the letter B 
will fall on the blind spot, and disappear. Resting the head on the 
right hand, with elbow on the table, gives about the right distance; if 
the B is still visible, move the head a little further from or nearer to. 
the paper. 
In the above, the distance of the blind spot from the yellow spot,. 
or point of regard, is twenty times the retinal distance; and the 
distance from the paper at which the B disappears is twenty times 
the value of & (distance of nodal point from retina) for the particular 
observer. 
