Tills instrument was designed to measure the small density areas met with on motion picture film which often cover not more than one-quarter of a square millimetre. It is based on the inverse square law and consists essentially of a small area light source moving in a vertical path between two small opal diffusing screens. The density to be measured is placed in contact with the upper diffusing screen. By an arrangement of mirrors, the lower diffusing screen, or comparison field is arranged to give the photometric field which is viewed by means of a low-power eyepiece. An endless steel tape attached to the light source carriage is graduated in terms of " density," so that direct density readings are obtained. The intensity scale that the instrument is capable of reading is from o to 3.o. Although the instrument was designed for motiort picture work, small density areas in negatives up to 12 Γ 15 inches can also be measured.
Production of Arsenic in 1923 . (From Circular of the Department of the Interior. )--The refined and crude arsenic produced in the United States in 1923, according to an estimate issued by the Department of the Interior through the Geological Survey, amounted to I4,I84 short tons, valued at $2,966,753, or an average value of io~ cents a pound. This quantity is a little less than that given in the estimate made in December. The sales of refined arsenic, having a purity of 99.5 per cent. or more, amounted to I2,I92 tons, and the sales of crude arsenic, having a purity of 7 Β° to 95 per cent., amounted to 1992 short tons. Considerable crude arsenic was used in the manufacture of weed killer and distributed to eastern and western railroads for use on their rights of way. The stock of domestic arsenic at points of manufacture was small; in fact, no more than 200 tons of refined arsenic was on hand October 3ist. During the first eleven morlths of the year 9168 short tons of white arsenic. valued at $1,695,654, were imported. Most of it came from Germany, Japan, and Mexico, but some came from Belgium, France, * Communicated by the Director.