LXXII.] DIRECT VISION. 339 



retina in the periphery of the blind spot behave as if two diametrically 

 opposite points approached each other. 



2. Map out the Blind Spot. 



Make a cross on the centre of a sheet of white paper, and place it on 

 a table about 10 or 12 inches from you. Close the left eye, and look 

 steadily at the cross with the right. 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 gradually move it to 



I- 



b c 



FIG. 257. Volkmann's Experiment on the Blind Spot. 



the right until the black becomes invisible. Mark this spot. Carry the 

 blackened point still farther outwards 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. 



3. Calculate the Size of the Blind Spot. 



Helmholtz gives the following formula for this purpose : When / is the 

 distance of the eye from the paper, F the distance of the second nodal 

 point from the retina usually 15 mm. d the diameter of the sketch of the 

 blind spot drawn on the paper, and D the corresponding size of the blind 

 spot : 



/ _ L. 



F ~ D 



4. Acuity of Vision of the Fovea Centralis. 



(a.) On a horizontal plane -a blackboard describe a semicircle with a 

 radius equal to that of the near point of vision, and fix in the semicircle pins 

 at an angular distance of 5 apart. Close one eye, and with the other look at 

 the central pin ; the pins on each side will be seen distinctly ; those at 10 

 begin to be indistinct, while those at 30 to 40 are not seen at all. 



(b.} At a distance of 5 feet look at a series of vertical parallel lines alter- 

 nately black and white, each .5 mm. wide. A normal eye will distinguish 

 them ; if not, approach the object until they are seen distinctly. 



5. Direct Vision. When the image of an object falls on the 

 fovea centralis, we have " direct vision." When it falls on any 

 other part of the retina, it is called " indirect vision." Vision is 

 most acute at the fovea centralis of the yellow spot. 



(a.) Standing about 2 feet from a wall, hold up a pen at arm's 

 length between you and the wall. Look steadily at a fixed spot 

 on the wall, seeing the pen distinctly all the time. Move the pen 

 gradually to one side ; first one fails to see the hole in the nib, and 

 as the pen is carried outwards one fails to recognise it as a pen. 



