Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . best solution to be hoped formust contain specifications of these two conditions. When thestage has been reached where one can give a definite candle-powerrating to a colored illuminant for a certain illumination the firststep will have been made. The second step will be when one 102 LIGHT, PHOTOMETRY AND ILLUMINATION can state also what the illuminants candle-power will be at any-other illumination, knowing it at the standard. The Whitman Flicker Photometer.—One of the earliest flickerphotomete
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . best solution to be hoped formust contain specifications of these two conditions. When thestage has been reached where one can give a definite candle-powerrating to a colored illuminant for a certain illumination the firststep will have been made. The second step will be when one 102 LIGHT, PHOTOMETRY AND ILLUMINATION can state also what the illuminants candle-power will be at any-other illumination, knowing it at the standard. The Whitman Flicker Photometer.—One of the earliest flickerphotometers was made by Whitman. Its construction is illus-trated in Fig, 46. The disk BD, constructed as shown at the left,revolves around its axis A. When in the position shown at theright it presents to the eye of the observer at 0 a surface illumin-ated by /j. When B has revolved from in front of the eye-piecethe observer sees the surface C which is illuminated by light from/j. In this way, by rotating the disk BD at its proper speed bymeans of a small motor or similar device, the two illuminated. Fig. 46.—The Whitman flicker photometer. surfaces are presented to the eye in rapidly alternating succession,and a balance is obtained by varying the intensity on one surfaceor the other as in the case of the ordinary type of photometerhead. The Simmance-Abady flicker photometer consists essentiallyof a thick disk of plaster of Paris driven by a clockwork mechan-ism. The edges of the disk are beveled as shown in Fig, 47 andthese beveled surfaces are viewed from above as they revolve bymeans of a 45 degree mirror or prism and the sight tube as shownin Fig, 48. It will be seen from the construction of the disksthat the observer sees first a surface illuminated by one lamp andthen a surface illuminated by the other presented alternately andin rapid succession when viewing the disk through the sight tubeand the prism. The speed of the disk can be adjusted ])y a regu- THE PRINCIPLES OF PHOT
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
