16 



HOW WE SEE 



are used, the colors cannot be distinguished 

 at threshold after this time (see Fig. 10). 

 It is from measurements such as these, 

 incidentally, that rod and cone threshold 



o 

 o 



o 



< 



X 

 I- 



10 



I I 



-pre-aoapting luminance 



10 20 30 40 



MINUTES IN THE DARK 



Fig. 9. The course of dark adaptation measured 

 for 101 subjects. The solid line shows the lu- 

 minance the average subject could see at various 

 times after the lights were turned out. The dotted 

 lines enclose the measurements for 95 percent of 

 the subjects. (Data from Sloan, 84) 



data similar to those in Fig. 7 can be 

 obtained. 



The instantaneous threshold exhibited 

 in Fig. 9 is an estimate from Blanchard's 

 data. Notice that the dark adaptation 

 curve itself extends over five log steps. 

 This illustrates the enormous range of 



adaptation of the eye since five log steps 

 correspond to luminances in the ratio of 

 100,000 to 1 . If the full range of sensitivity 

 of the eye is measured — from the pre- 

 adapting luminance to the final rod threshold 

 — the range is about eight log steps, or 

 luminances in the ratio of 100,000,000 to 1 ! 



An Inconsistency in Measurement. At 

 this point we may indicate an inconsistency 

 in concepts which is implied in the measure- 

 ment of dark adaptation and, in fact, in all 

 the data of this section whenever thresholds 

 are reported (Fig. 4, for example). Light 

 has been defined as "radiant energy . . . 

 evaluated according to its capacity to 

 produce visual sensation." But a stimulus 

 below threshold, i.e., a stimulus which 

 cannot be seen, does not arouse a visual 

 sensation and so cannot properly be called a 

 light. Yet most measurements of dark 

 adaptation express the intensity of the 

 stimulus in luminance measurements and 

 show the scale extending below the visual 

 threshold. The absolute sensitivity of the 

 human eye must define the zero point of a 

 luminance scale. For most practical pur- 

 poses, this is a minor difficulty, but it is 

 further evidence of the need for a consistent 

 system of low-luminance photometry. 



How Long Does it Take for Complete Dark 

 Adaptation? A matter of some practical 

 importance to the armed services is the 

 length of time an individual must remain in 

 darkness to reach maximum visual sensi- 

 tivity. During the last war, this problem 

 was important enough in certain types of 

 military jobs, e.g., night lookout duty and 

 night flying, for instructions to be issued 

 requiring the men concerned to adapt for 

 prescribed periods of time before reporting 

 for duty. Many night-vision manuals "writ- 

 ten during this period stated categorically 

 that 30 minutes in the dark were required to 

 reach maximum visual sensitivity. This 

 statement requires considerable amendment. 

 In the first place, no experimental study to 

 date has followed dark adaptation to 

 completion. Semeonoff (80), however, stud- 



