VISUAL ACUITY 



125 



the angle at the eye, subtended by the interspace, must be greater than 

 a certain minimum value. This angle is very close to 1 minute of arc, 

 although values as low as 30 seconds have been obtained for persons of 

 exceptionally good vision. Taking the posterior nodal point of the lens 

 as 16.7 mm from the retina, 1 minute of arc corresponds with a distance 

 of 0.0048 mm between the images at the retina. The diameter of the 

 outer segment of a cone in the fovea is 0.002 mm, and its inner segment, 

 which is a close-packed hexagonal mosaic, has a diameter of 0.003 mm; 

 it follows that two points on an image can straddle a single cone and 

 still be 0.0048 mm apart. Two point images can, therefore, be resolved 

 by the mosaic of the fovea if one unilluminated cone lies between them. 



E. Hering in 1900 advanced the following 

 interpretation in support of the mosaic-pattern- 

 structure theory. Figure III— 14 shows a scale 

 drawing of the retinal mosaic on which an 

 image with a broken line of separation between 

 light and dark portions is indicated. It is as- 

 sumed that a change in stimulation of a single 

 cone or column of cones is necessary to perceive 

 the displacement of a retinal image. In image 

 a the upper half of the cells in column c is stim- 

 ulated, but in the lower section no cell of this 

 column is stimulated. Hence a break in the 

 edge of the image should be perceived. In b the 

 image is shown slightly displaced to the right. 

 No discontinuity can be perceived in its edge 

 because all the cells in column d are stimulated. 



The upper diagram in Fig. Ill— 14 indicates 

 that the horizontal displacement of the broken 

 edge of the image can be as small as x and still 

 excite two parallel columns of cells. If x is as 

 small as 0.00087 mm for the hexagonal close- 

 packed cone system in the retina, then x sub- 

 tends an arc of 12 seconds at the nodal points re ti na , 

 of the eye, which are at an average distance 



of 16.8 mm from the retinal image. The results are supported by the 

 experimental evidence. It has been shown experimentally that the 

 minimum separation of a break in a line detectable by unaided vision 

 is as low as 12 ± 4 seconds of arc. 



The conclusion is, therefore, that visual acuity for the positions and 

 movements of contours is about five times greater than it is for resolu- 

 tion of double points and lines. Therefore, where interpolation methods 



Fig. 111-14. Hering's 

 mosaic pattern of the 



