. Contrast sensibility of the eye . s fact. V. DATA OBTAINED The two methods by which the two series of observations pre-viously mentioned were obtained are as follows: 1. The illumination on the screen (this will be referred to asthe field brightness) is kept constant, while the illumination onthe strip (strip brightness) is varied by known amounts by means 2 Trans. 111. Eng. Soc. 11, p. i; 1916. 686 Scientific Papers of the Bureau of Standards [Vol. 15 of sectored disks. By this method a relation is directly obtainedbetween the length of strip and contrast between the field andstrip for any
. Contrast sensibility of the eye . s fact. V. DATA OBTAINED The two methods by which the two series of observations pre-viously mentioned were obtained are as follows: 1. The illumination on the screen (this will be referred to asthe field brightness) is kept constant, while the illumination onthe strip (strip brightness) is varied by known amounts by means 2 Trans. 111. Eng. Soc. 11, p. i; 1916. 686 Scientific Papers of the Bureau of Standards [Vol. 15 of sectored disks. By this method a relation is directly obtainedbetween the length of strip and contrast between the field andstrip for any desired field brightness. 2. The relative brightness of the field and strip is fixed, and thebrightness of both is reduced in known steps by sectored relation is thus obtained directly between the length of stripand the field brightness for any desired contrast between fieldand strip. Both of these methods differ materially from the method bywhich Reeves 3 obtained his excellent results, and also differ fromthat of IZO STRIP LENGTHS) VISUAL ANGL£(OCG) Observer, E. P. T. T., June 20, 21, 1919 Fig. 4.—Relation between strip length (visual angle) and ratio of brightness difference to field brightness Average of decreasing and increasing brightness difference: 1, F. microlamberts; 2, F. ; 3, F. 160 microlamberts 1. RESULTS BY FIRST METHOD The data obtained by the first method for each of the twoobservers are given in curves of Figs. 4 and 5 where the strip lengthis indicated along the axis of abscissae in centimeters, and interms of the angle which this length subtended at the observers 1 J. Opt. Soc. Amer., 1, p. 148; 1917. Physiological Optics, p. 135. The results of Blanchard should also be noted, obtained by a similarphotometric method and confirming the results of Koenig, when the intensity unit of the latter are properlyevaluated. These are reported by Nutting in Trans. 111. Eng. Soc., 11* p. 939,1916; and in J. Frankl. In
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