26 



HOW WE SEE 



by keeping the form of the test object 

 constant. The test factor, the position 

 of the break in the Landolt ring (or the 

 orientation of the parallel bars), is changed 

 from trial to trial. A series of rings or 

 bars, varying in size, is used to measure 

 acuity. Many other kinds of test objects 

 have been used to measure acuity, and 

 some of them are described in a report of 

 the Personnel Research Section of the 

 Adjutant General's Office (103). 



Visual Angle. An index or measure of 

 visual acuity which the laboratory scientist 

 prefers to the 20/x notation is the size of 

 the smallest detail an observer can see in 



of visual angle (38). This is roughly 

 equivalent to seeing a wire, reth inch in 

 diameter, a half mile away ! This, however, 

 is more a measure of visibility — about 

 which we shall say more later — than of 

 visual acuity. 



Physiological Factors Affecting Acuity 



Near and Far Acuity. A very important 

 factor which determines visual acuity is the 

 distance at which the test is made. At 

 first glance, this might not appear reasonable 

 if the visual acuity is expressed in angular 

 measurements. Actually, experimental evi- 

 dence shows that visual acuity at 13 inches 



SNELLEN 



Fig. 16. Three types of test object used in the measurement of visual acuity, 

 there are actually many critical details to be seen. (After Crouch, 23) 



In the Snellen letter 



terms of the visual angle subtended by this 

 object at the eye (see Fig. 16). This kind of 

 measure is convenient because it makes no 

 assumptions about the visual acuity of a 

 hypothetical normal person, and it gives us a 

 single number to work with. Snellen rat- 

 ings of acuity are usually translated into 

 measures of acuity expressed in visual 

 angles by an arbitrary conversion factor, 

 viz., a Snellen rating of 20/20 corresponds 

 to a visual angle of one minute. 



Acuity as measured by the test forms in 

 Fig. 16 does not represent the maximum 

 acuity of the eye. Recent measurements 

 show, for example, that under ideal condi- 

 tions the eye can detect the presence of an 

 object which subtends about a half second 



may be quite different from acuity at 20 

 feet (29, 85). Acuity for distances greater 

 than 20 feet appears to remain fairly 

 constant. The reason for this is that the 

 lens in the front of the eye has to change 

 shape in order to focus near and far objects 

 upon the retina (accommodation), and 

 people differ markedly in their ability 

 to do this. Accommodation remains fairly 

 constant for objects at distances greater 

 than 20 feet. 



This fact is of some practical importance. 

 In some industries, and in many military 

 situations, men are selected for particular 

 jobs on the basis of results obtained in 

 physical examinations. Because visual 

 acuity is related to accident-proneness and 



