The new international encyclopaedia . n the sur-faces of the prism C. The two lights are movedso that the surfaces of the prism appear equallyilluminated to the eye of the observer at E. Asimple and ellective photometer was devised byLambert in 1700, but as it was used extensivelyby Kumford some years later, it is known by hisname. It consists of a vertical screen with awhite surface, in front of which stands a cy-lindrical stick. The lights to be compared areplaced in front of this stick, so that each willcast a shadow on the screen. These shadows rep-resent the amount of light furnished by h
The new international encyclopaedia . n the sur-faces of the prism C. The two lights are movedso that the surfaces of the prism appear equallyilluminated to the eye of the observer at E. Asimple and ellective photometer was devised byLambert in 1700, but as it was used extensivelyby Kumford some years later, it is known by hisname. It consists of a vertical screen with awhite surface, in front of which stands a cy-lindrical stick. The lights to be compared areplaced in front of this stick, so that each willcast a shadow on the screen. These shadows rep-resent the amount of light furnished by hut oneof the sources, consequently by varjing tlie dis-tances of the lights from the screen a positioncan be obtained where they furnish shadows ofe<|ual intensity. Measuring the distance of thelights from the screen, squaring these numbers,and forming a proportion, we can again compare the intensity of the two illuminants. A with an aperture cut iu it may be placedparallel to the first with the lights in front of it. Fia. 3. BCMFOKD PHOTOMETEB. and some little distance apart, so that each il-luminates the screen by means of rays sentthrough the aperture. The two separate spots oflight on the screen can be made equal Ijy adjust-ing the lights and their comparative intensitycomputed as before. The photometer in most gen-eral use is that of Bunsen, where the two lightsto be compared, A and B, are placed at the A*- -*B Flo. 4. BUNSEN PHOTOMETER. opposite ends of the apparatus and a screen, C,whose face is perpendicular to the incident rayscan be moved to and fro between them. Thisscreen is made of a sheet of white paper with acentral spot, d, produced by oil or grease, whichappears bright on a dark ground by transmittedlight, and dark on a light ground by reflectedlight. When the transmitted and reflected lightsare equal in intensity the spot should disajipearand the surface of the paper appear uniform, butin aetmil practice this condition is not shee
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