. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . consider the light of the solid white rings as 360. The differencebetween the extreme gray rings and the white will be 5 and the rela- 81 5 1 tion — will then be equal to = — which represents the value of I 360 72 226 PHYSIOLOGICAL OPTICS Feehners fraction for the patient under examination if he candistinguish the three images. If he can distinguish only two, Feehners 360 — 350 1 fraction becomes = —. A large number of sectors is 360 36 used in such experimentation; good illumination must be emp
. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . consider the light of the solid white rings as 360. The differencebetween the extreme gray rings and the white will be 5 and the rela- 81 5 1 tion — will then be equal to = — which represents the value of I 360 72 226 PHYSIOLOGICAL OPTICS Feehners fraction for the patient under examination if he candistinguish the three images. If he can distinguish only two, Feehners 360 — 350 1 fraction becomes = —. A large number of sectors is 360 36 used in such experimentation; good illumination must be employedand the patient must not be too remote in order that the influence ofdiminished visual acuity (because of distance) may not enter intothe problem. 186. A simple modification of this rotating disc, made by Masson,consists in drawing upon a white disc a series of equal black and whiteapertures running in a straight line from the center of the disc to theperiphery as shown in Fig. 116. Upon rotation the black lines formgray bands which vary in distinctness as the center is Fig. 116.—Modification of Disc of Masson. In order to determine the minimum perceptible difference one mustcount the gray and white rings, produced by the rotation of the disc,proceeding from the center to the circumference. The distance whichseparates the center of the disc from the black mark which formsupon rotation the last discernible gray circular ring is proportionalto the light sense of the subject examined. The value of the minimumdifferential can thus be obtained as a fractional part of the brightnessof the white disc; if d is the width of the black rings, i. e., the size ofthe black interruptions on the disc as represented in Fig. 116, and ris the radius of the last discernible gray ring, when the disc is rotated,measured from the center of the disc, and if the intensity of the whiteportions of the disc is taken as unity, then the intensity h of a grayband during rotation i
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