Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . e left side of the wedge. Theinfluence of the prism Z^ is the same as Z^ except that it is 90degrees displaced. The result is that the field of view is dividedinto four quadrants, with opposite quadrants illuminated bythe same source. In the equality of brightness photometer thephotometrical l)alance is obtained when the four quadrants areof equal brightness. In the contrast photometer the contrastingelement may consist of two thin squares of glass, K^ and K^,Fig. 37, inserted in the path of the
Light, photometry and illumination : a thoroughly revedof ''Electrical illuminating engineering'' . e left side of the wedge. Theinfluence of the prism Z^ is the same as Z^ except that it is 90degrees displaced. The result is that the field of view is dividedinto four quadrants, with opposite quadrants illuminated bythe same source. In the equality of brightness photometer thephotometrical l)alance is obtained when the four quadrants areof equal brightness. In the contrast photometer the contrastingelement may consist of two thin squares of glass, K^ and K^,Fig. 37, inserted in the path of the rays illuminating oppositequadrants, as shown in Fig. 38a. This glass absorbs a small 92 LIGHT, PHOTOMETRY AND ILLUMINATION amount of light, the right amount to produce the desired con-trast. The unbahinced condition of the field is shown in Fig. selenium photometer takes advantage of the fact that theelectrical condition of selenium changes under the influence oflight. This property of selenium makes it especially fit for theconstruction of photometers, although nearly all attempts to.
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1912
