VISUAL ACUITY 



25 



curves show clearly that at certain lumi- 

 nance levels, best visual acuity occurs about 

 4° from the fovea. Confirming data come 

 from an experiment by Gordon (30), who 

 finds the most acute region of the retina at 

 rod levels to be between 5° and 7°. 



These data are of practical value, of 

 course, because they show that we see things 

 best at very dim luminances by looking 

 about 5° to one side of them. At rod 

 levels one cannot see things well by looking 

 directly at them. A second, and for 

 theoretical reasons a more important, im- 

 plication of these data is that they provide 

 additional evidence for the conclusion that 

 dark adaptation and night vision (at least 

 visual acuity at dim luminances) are 

 different functions. The writer can advance 

 no good physiological explanation to account 

 for this state of affairs, but it must be 

 associated in some way with the rod popula- 

 tion and neural interconnections we spoke 

 about earlier. 



Conclusions. A general conclusion which 

 can be drawn with some assurance from this 

 evidence is that night vision is a complex 

 visual process. Dark adaptation is one of 

 the factors which contributes to night 

 vision but there are other factors which 

 may be as important, or more important, in 

 determining night visual capacity. What 

 these other factors are, and how much they 

 contribute to night vision, is not completely 

 understood. The fact that dark adaptation 

 and night vision appear to be largely 

 independent is of practical importance not 

 only in designing night vision tests, but 

 also in instnicting men engaged in night 

 duties. 



Visual Acuity 



The Measurement of Visual Acuity 



Visual acuity, in its broadest definition, 

 means the ability to distinguish fine detail. 

 In actual practice there are many different 

 ways in which acuity is measured and 

 expressed. We shall discuss some of the 

 more common techniques briefly here. 



Snellen Ratings of Acuity. Perhaps the 

 most common and familiar method of 

 testing visual acuity is by means of a doctor's 

 eye chart. The typical chart of this sort 

 contains a series of letters of various sizes, 

 and the examinee reads as many as he can. 

 The smallest detail an observer can see is a 

 measure of his acuity (see Fig. 16). When 

 acuity is measured in this way it is usually 

 expressed as a ratio of the distance at 

 which a given line of letters can be seen by 

 the examinee to the distance at which a 

 person with normal vision can see the same 

 letters. Thus, a rating of 20/20 means 

 that the examinee could see at 20 feet 

 letters which the normal person can read 

 at 20 feet. A rating of 20/40 means the 

 examinee could just read at 20 feet what the 

 normal person can see at 40 feet. A rating 

 of 20/10 means that the examinee could see 

 at 20 feet what the average person can just 

 barely see at 10 feet. 



Actually, these ratings are ambiguous 

 because they depend greatly on the defini- 

 tion of what a "normal" person can see. A 

 rating of 20/20 is usually taken as the 

 normal rating, but, as we shall see, this 

 varies greatly depending on the amount of 

 illuminance, the contrast between the object 

 and its background, etc. 



Two other kinds of visual acuity test 

 objects are also shown in Fig. 16. These 

 are the Landolt ring and parallel bars. 

 Both of these test objects can be used with 

 illiterates, of course, and so have an ad- 

 vantage over the Snellen letters for such 

 subjects. A much more important ad- 

 vantage, however, is that they largely 

 eliminate a form perception factor which 

 vitiates results obtained with letter tests 

 (103). Even though a series of letters may 

 have the same overall size, the same thick- 

 ness of lines, and so on, some of them will 

 be more legible than others simply because 

 of their form; e.g., the C, 0, and G are 

 easily confused whereas the T and L are 

 fairly distinctive. The Landolt ring and 

 parallel bars eliminate this perceptual factor 



