28 



HOW WE SEE 



of the retina nearer the nose, and is about 

 seven degrees wide and about five degrees 

 high. 



Since there are no rods or cones in the 

 bhnd spot, it is hterally bhnd. You 

 cannot see anything in this area; and yet if 

 you try to find a hole somewhere in your 

 visual field, you probably cannot find one. 

 It is there, nonetheless, and any laboratory 

 psychologist could demonstrate it very 

 simply. 



Physical Factors Affecting Visual Acuity 



In contrast to the previous section, our 

 concern here is how visual acuity is influ- 



values above the cone threshold, visual 

 acuity was measured for foveal or central 

 vision. This curve illustrates several im- 

 portant points. First, notice how rapidly 

 visual acuity increases as the luminance 

 increases in the middle range of values — 

 from 0.01 to 100 mL (log = -2 to log = 2). 

 Beyond this value visual acuity does not 

 increase so much, although the curve 

 continues to rise indefinitely, even if very 

 slowly, as the luminance increases. 



Although the curve in Fig. 18 is the kind 

 frequently used in discussions on vision, 

 it is important to note that there are 

 alternative ways of plotting the same data 



oo 



BACKGROUND LUMINANCE IN LOG MILLILAMBERTS 



Fig. 18. Visual acuity as a function of background luminance. (From data compiled by Moon and 



Spencer, 69) 



enced by physical factors in the environ- 

 ment. There are so many research studies 

 on these factors that it would be impossible 

 to review them all here. It has been 

 necessary, therefore, to select representative 

 data to illustrate each point. 



Luminance and Visual Acuity. We have 

 already had occasion to note the importance 

 of the first factor, viz., luminance (see 

 Fig. 15). The curve in Fig. 18 is an average 

 curve drawn through the data of six thor- 

 ough investigations on visual acuity as a 

 function of background luminance. For 



and that these alternative methods give 

 slightly different results. The scale at the 

 left in Fig. 18 is a scale of visual acuity, 

 defined as the reciprocal of visual angle in 

 minutes. If one compares this scale with 

 the visual angle scale at the right in the 

 figure, it becomes immediately apparent 

 that the visual acuity measure, by its 

 very nature, yields a distorted scale. A 

 change in visual acuity from 0.5 to 1.0 

 means a change of one minute in the size 

 of the smallest discriminable object, i.e., 

 from 2.0 to 1.0 minutes. A similar change 



