224 LABOR A TOR Y G UIDE IN PHYSIOL OGY. 



cross and a circle about three inches apart. Closing 

 the left eye hold the card vertically about ten inches 

 from the right eye and so as to bring the cross to the 

 right side of the circle. Look steadilyat the cross with 

 the right eye, when both the cross and the circle will 

 be seen. Gradually bring the card toward the eye, 

 keeping the axis of vision fixed on the cross. At 

 a certain distance the circle will disappear, /. e., when 

 its image falls on the entrance of the optic nerve. On 

 bringing the card nearer, the circle reappears, the 

 cross of course being visible all the time. 



(10) Map out the blind spot. 



Make a cross on the center of a sheet of white paper 

 and place.it on a table about ten or twelve inches from 

 you. Close the left eye and look steadily at the cross 

 with the right eye. Wrap a penholder in white paper, 

 leaving only the tip of the pen point projecting, dip 

 the latter in ink, or dip the point of a white feather in 

 ink, and keeping the head steady and the axis of vision 

 fixed, place the pen point near the cross and gradu- 

 ally move it to the right until the black becomes in- 

 visible. Mark this spot. Carry the blackened point 

 still further outward until it becomes visible again. 

 Mark this outer limit. These two points give the 

 outer and inner limits of the blind spot. Begin again 

 moving the pencil first in an upward and then in a 

 downward direction, in each case marking where the 

 pencil becomes invisible. If this be done in several 

 diameters an outline of the blind spot is obtained, 

 even little prominences showing the retinal vessels 

 being indicated. 



(11) To calculate the size of the blind spot. 

 Helmholtz gives the following formula for this purpose: 

 When f is the distance of the eye from the paper, F 



