VISUAL ACUITY 



27 



efficiency in certain types of jobs, this is a 

 critical item in the physical examination. 

 This means, then, that if a job calls for good 

 distance acuity, e.g., truck driving, the men 

 should be tested at 20 feet. But if the job 

 calls for good near acuity, e.g., toolmaking 

 or operating a radar, then the men should 

 be tested at a distance comparable to that 

 used on the job. Because all other factors 

 which affect acuity seem to affect near and 

 far acuity alike, we shall not have to talk 



illustration, zero degrees represents the 

 straight-ahead direction — sometimes called 

 the line of sight. Light rays from this 

 direction strike the fovea — the center part 

 of the eye. Notice that visual acuity drops 

 off rapidly for objects in the periphery of 

 the eye. As a matter of fact, when you 

 look straight ahead, your visual acuity 

 five degrees to the right or left of the 

 central line of sight is just half as good as 

 it is in the fovea. At 40, 45, and 50 degrees 



3 

 O 

 < 



UJ 



> 



70* 60* SO* 40* 30* 20* I0« 0* 

 NASAL BLIND 



SPOT 



0* 20* 3 

 TEMPORAL 



DEGREES FROM THE FOVEA 

 Fig. 17. Visual acuity at different retinal positions. (After Wertheim, 97) 



more about distances in the following 

 discussion. 



Visual Acuity and Retinal Location. We 

 all use our eyes so much that we commonly 

 overlook certain peculiarities about our 

 seeing process and would never notice 

 them if they were not pointed out to us. 

 If, for example, you stare steadily at a 

 letter on this page, it is impossible for you 

 to read letters two inches away; or if you 

 look at the road straight ahead, while 

 driving, you will not be able to read most 

 signs along the side of the road. The 

 variation of visual acuity in different parts 

 of the eye is plotted in Fig. 17. In this 



from the central line of sight, visual acuity 

 is only about ^V of what it is directly 

 straight ahead. It is interesting to note, 

 incidentally, how closely this curve parallels 

 the distribution of cones in different parts 

 of the retina (see Fig. 2). 



The Blind Spot. One thing more before 

 we leave Fig. 17. Notice that there is a 

 blacked-off area in the figure. This is 

 labelled the "blind spot." This area was 

 also marked off in other figures we have 

 used so far (see Figs. 3 and 4). The blind 

 spot is the place where the nerves and blood 

 vessels come into the eyeball. It is located 

 about 15 degrees from the fovea, on the side 



