20 



HOW WE SEE 



data of Haig are supported by the findings 

 of a number of other studies (36, 70, 98), 

 they are all fragmentary in one respect or 

 another. Haig's study, as we noted, was 

 done on one normal and one pathological 

 subject. In his study of the effect of the 

 pre-adapting intensity, he used only one 

 exposure time, four minutes, which, accord- 

 ing to his own data (Fig. 11), is not sufficient 

 for full light adaptation. In his study of the 

 effect of exposure time, he used only one 

 adapting value, 447 mL. Virtually the 

 same kinds of objections can be raised about 

 every other study on this problem. As a 

 result, it is impossible to obtain the critical 

 values needed for any practical specification 

 of ready-room illumination. 



Further research on this problem would be 

 of immense practical value. In particular, 

 the following points should be observed: 



(a) A greater number of subjects should 

 be used to obtain better normative data. 



(b) The effect of pre-adapting luminance 

 should be studied with relatively long 

 durations of pre-adaptation to insure com- 

 plete light adaptation. Wald and Clark 

 (92) and Miiller (70) find that no further 

 change in dark adaptation is evident if 

 pre-exposures exceed 10 minutes. Here 

 again, however, their studies were done with 

 single luminance levels. Whether this is 

 true for other luminance levels constitutes 

 another problem in itself. If the subjects 

 were completely light-adapted, the situation 

 would be more comparable to that found in 

 service conditions where men may sit around 

 a ready-room or ward room for long periods 

 of time. 



(c) The effect of duration of exposure 

 should be reinvestigated with a greater 

 range of Imninances. 



(d) Haig believes that, within certain 

 limits, the luminance of the pre-exposure 

 fight and the duration of the pre-exposure 

 may be interchangeable. If this is so — 

 and it seems entirely reasonable on the basis 

 of other visual functions in which intensity 

 and time may be interchanged — it would 



provide a single measure of "amount of 

 fight adaptation" which could be specified 

 as a function of intensity and time. Amount 

 of light adaptation, in turn, could then be 

 related to the length of time required for 

 subsequent dark adaptation. It should be 

 evident that this conjecture is weU-supported 

 by our present data, but that these data 

 are far too few to be put to any practical use. 



With data of such investigations at hand, 

 visual scientists would be able to specify 

 with some certainty the length of time men 

 must dark-adapt after having been in 

 ready-rooms with known amount of illu- 

 minance. Present data suggest that this 

 period will be of the order of a few minutes. 

 The proposed new studies would also show 

 how serious it would be for dark-adapted 

 men to turn on dim lights from time to time. 



Independence of Adaptation in the Two 

 Eyes. It is now known that dark-adapta- 

 tion proceeds independently in each eye. 

 This fact is of considerable importance to 

 the soldier or sailor because it means that if 

 he must turn on a light for some reason he 

 can close one eye to preserve the dark 

 adaptation in that eye. When he turns the 

 light off, the dark adaptation of the eye that 

 had been closed is virtually undisturbed. 



Color of Pre-Adapting Light. Still another 

 important determinant of the rate of dark 

 adaptation is the color of the pre-adapting 

 light. If pre-adaptation is carried out with 

 red light, dark adaptation proceeds very 

 rapidly after the pre-adapting light is 

 turned off. If, on the other hand, pre- 

 adaptation is done with white, green or blue 

 light, dark adaptation proceeds much more 

 slowly. Although the validity of this 

 relationship was challenged by Lowry (55) 

 during the war, careful studies by Hecht and 

 Hsia (37) and Rowland and Sloan (79) 

 showed Lowry to be wrong. We may, 

 therefore, accept it as verified fact. 



The Use of Red Goggles. Red goggles 

 were widely used during the last war to 

 dark-adapt the eyes. Their use was so 

 common that our discussion of this problem 



