. Contrast sensibility of the eye . the Bureau of Standards [Vol. 15 vations of the strip length when just visible, both upon decreasingand upon increasing it. The strip length for the former conditionwas usually very much less than that for the latter. Such obser-vations with decreasing length of strip were discontinued, however,for the following reasons: 1. The results obtained on decreasing the strip were far moreerratic than those obtained on increasing the strip. The lattercould be quite satisfactorily duplicated from day to day whenconditions were the same. The visibility of the strip wh
. Contrast sensibility of the eye . the Bureau of Standards [Vol. 15 vations of the strip length when just visible, both upon decreasingand upon increasing it. The strip length for the former conditionwas usually very much less than that for the latter. Such obser-vations with decreasing length of strip were discontinued, however,for the following reasons: 1. The results obtained on decreasing the strip were far moreerratic than those obtained on increasing the strip. The lattercould be quite satisfactorily duplicated from day to day whenconditions were the same. The visibility of the strip while de-creasing was more a matter of attending to the movement of the. 20 40 go eo too 120 STRIP LENGTH (Cm OA a* 21 2(> VISUAL ANGLE COB) Fig. Observer, E. between field brightness and strip length (visual angle) ratio betweenbrightness difference and field brightness terminal boundaries of the strip even when such movement wasslow. 2. The time required was approximately doubled so thatgreater fatigue effects were introduced into the observations thatwere reliable. 3. Much longer time than could be devoted to these experi-ments would have been demanded in order to obtain satisfactoryresults with the strip decreasing. The curves of Figs. 4 and 5 are similar, and are somewhat sug-gestive of hyperbolas, but by test are shown not to be curves are typical of curves that are obtained under ap- Karrer jTyndall] Contrast Sensibility of the Eye 691 proximately the same external conditions. These data should notbe generally applied, but similar curves obtained for a large num-ber of observers would be applicable. The necessity of
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