. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. ;Obliged to contrive a new photometric principle. In fact he solved ?the problem in a very ingenious manner. If a disk be divided into sectors equally large ^'"- "^• and alternately black and white, as in fig;. 79, ;5ind be put into rapid rotation, the different sectors cannot be distinguished when the disk is illuminated by a constant source of light; but if it be illuminated by an electrical spark for an instant the sectors of t
. Annual report of the Board of Regents of the Smithsonian Institution. Smithsonian Institution; Smithsonian Institution. Archives; Discoveries in science. ;Obliged to contrive a new photometric principle. In fact he solved ?the problem in a very ingenious manner. If a disk be divided into sectors equally large ^'"- "^• and alternately black and white, as in fig;. 79, ;5ind be put into rapid rotation, the different sectors cannot be distinguished when the disk is illuminated by a constant source of light; but if it be illuminated by an electrical spark for an instant the sectors of the rotating disk iwill become visible again, and as much more ,so as the electrical spark is brighter. But if |the illumination by the electrical spark be gradually weakened, while that from the con- stant source of light remain the same, a point will be attained where the sectors just cease to be distinguishable, and in this case the power of the illumination by the electrical spark is a determinate fraction of the illumination by the constant source, its magnitude depending lupon the peculiarity of the observer's eye. J We will now consider in what manner this limit of the ability to !distinguish may be ascertained. A part of a sector on a white disk, (fig. [80,) being blackened, and the disk turned rapidly about its centre, the black piece will form a ring somewhat darker than the white ground of the disk. The ring will appear as much fainter as the black spot is narrower, , and if the experiment be made with a series of such disks, each successive one having a narrower black end-portion of a sector m n, I we will at last fin«! one in which the dark ring ceases to be distinguishable. Xet us suppose this to be the case when I the breadth of the sector is y^-q of ^be entire .circumference; it is evident that the brightness of the ring is less than the brightness of the disk by jl--^ ; in this case the eye cannot j distinguish a difi'erence of yio- ^^ illumination. Masson ma
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