274 EXPERIMENTAL PHYSIOLOGY. 



3. To Calculate the Size of the Blind Spot. 



(a.) Helmholtz gives the following formula for this pur- 

 pose : 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- d 



F D 



4. Acuity of Vision of the Fovea Centralis. 



(a. ) On a horizontal plane a blackboard describe a semi- 

 circle 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 five feet look at a series of vertical 

 parallel lines alternately 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 what is called " direct vision," but when 

 the image falls on any other part of the retina, it is called " in- 

 direct vision." Vision is most acute at the fovea centralis of the 

 yellow spot. 



(a.) Standing about two 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. Hence, in looking at a large 

 surface, to see it distinctly one must unconsciously move his 

 eyeballs over the surface to get a distinct impression thereof. 



(b.) Make two black dots on a card quite close together 

 so that when looked at they are seen as two. Hold up the 

 left index finger, look steadily at it, and place the card with 



