24 



HOW WE SEE 



Pinson and Chapanis (74), Rowland and 

 Mandelbaum (77), and Chapanis (6) are 

 some of the other authors who have pertinent 

 data on this point. 



Since it is not generally realized how 

 many such data exist, it would be a worth- 

 while project for someone to collect these 

 obscure war-time reports and collate their 

 findings. In many cases, the numbers of 

 subjects were few, but the aggregate 

 findings of these studies constitute legitimate 

 data and their conclusions cannot be ignored. 



I 

 09 



08 



V 



t 07 



o 



-^ 06 



_J 

 < 



to 05 



> 



04 



03 



02 



BACKGROUNO LUMIiNiANCE IN LOG pp.L 



Fig. 15. Visual acuity as a function of luminance 

 at various retinal locations. Notice that at 

 certain low luminance levels, best acuity occurs 

 about 4° from the fovea. (Data from Mandel- 

 baum and Rowland, 64) 



Tests of dark adaptation do not appear to 

 correlate well, if at all, with tests requiring 

 the observer to locate or identify forms at 

 rod levels of luminance. This conclusion 

 is valid for individuals with normal visual 

 capacities. If night-blind or pathological 

 cases are included, the correlations are a 

 little better because of the increased hetero- 

 geneity of the data (in a purely statistical 

 sense) . 



A Given Level of Adaptation Does Not 

 Giiarantee Acuity. This point is a difficult 



one to make because it is almost impossible 

 to find data on it. Briefly, however, it is 

 this: during the course of dark adaptation, 

 when the sensitivity of the eye has increased 

 so that it can perceive light of a certain 

 luminance, it cannot discriminate forms at 

 that luminance level. Adaptation must 

 continue beyond this point before the eye 

 can see things. Miles' data (66) — on only 

 one case — illustrate the point. His subject 

 was able to see a surface with a luminance of 

 five log /x/xL after about 9.5 minutes of dark 

 adaptation, but he could not see a silhouette 

 with a background of the same intensity 

 until six minutes later. Low (53) reports 

 confirming data, and concludes that "Maxi- 

 mal form acuity follows light perception by a 

 measurable interval, sometimes as long as 

 15 minutes. . . . Retinal sensitivity to light 

 of a given intensity does not guarantee 

 maximal visual powers." Low, unfortu- 

 nately, does not report the intensity of the 

 lights he used, nor is it possible to calculate 

 the luminance levels from the data he gives. 

 This, of course, is a very important requisite 

 for a fuller understanding of the process. 

 Needless to say, further work is needed to 

 elucidate with more precision the relation- 

 ship between these two aspects of visual 

 performance. 



Retinal Variations in Scotopic Visual 

 Acuity. Another very important difference 

 between dark adaptation and night vision 

 concerns the region of maximum sensitivity 

 and visual acuity on the retina. As we 

 have already seen (Figs. 3 and 4), the region 

 of maximum sensitivity to light occurs 

 about 20° from the fovea. This is the basis 

 for the statement, heard many times during 

 the war, that to see things best at night we 

 must not look at them but must look at a 

 point about 20° away from them. Actual 

 experimental data on visual acuity at rod 

 levels, however, show that the region of the 

 retina which is most acute is considerably 

 closer to the fovea. Fig. 15, for example, 

 shows the data obtained in a study by 

 Mandelbaum and Rowland (64). These 



