. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . plate B which serves forobservation; the minimum aperture of the diaphragm which permitsthe observer to distinguish the plate B determines the threshold. A 225 PHYSIOLOGICAL OPTICS modified form of the instrument, known as the differential photoptom-eter and which is particularly serviceable when the intensities are 81 feeble and for the determination of —, consists of two photoptometers I at right angles to each other and each making an angle of 45° with aglass plate placed in the center of the
. Physiological optics : being an essay contributed to the American encyclopedia of ophthalmology . plate B which serves forobservation; the minimum aperture of the diaphragm which permitsthe observer to distinguish the plate B determines the threshold. A 225 PHYSIOLOGICAL OPTICS modified form of the instrument, known as the differential photoptom-eter and which is particularly serviceable when the intensities are 81 feeble and for the determination of —, consists of two photoptometers I at right angles to each other and each making an angle of 45° with aglass plate placed in the center of the long tube containing the firstphotoptometer and the sighting tube. The instrument is assembledin a manner analogous to the first ophthalmoscope of Helmholtz. 185. Another method of determining the smallest perceptible dif-ference is due to Masson and depends upon the principle of persist-ence of vision. The disc of Masson is a white one in which differentsectors of varying sizes have been blackened as shown in Fig. causing a sufficiently rapid rotation of this disc one can see three. Fig. 115.—Disc of Masson. x& gray rings separated by white intervals. Supposing the sector a is 20°and the sectors b and c to be 10° and 5° respectively, and furtherassuming that the black areas do not reflect any light at all, thebrightnesses of the three gray rings will be 340, 350 and 355 if weconsider 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 visu
Size: 1591px × 1571px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, booksubjectphysiologicaloptics
