STANDARDS OF LIGHT. The question of stand ards of light possesses an intimate and impor tant connection with photometry. and is a sub ject which is still attracting nmeh attention from physicists. Until the International Congress on Electrical Units in ISti3 adopted as the absolute standard the amount of light emitted by a square centimeter of melted platinum at the point of solidification, there was no really scientific unit in general use. Numerous forms of candles and lamps were employed. but little reliability could he placed on the results obtained in such tests and they did not form a satisfactory basis for subsequent or independent. determinations. In France the standard used in photometric tests Was the CalTel MDT, which burned purified rape seed oil. The lamp was constructed according to certain fixed specifications, but it did not ap peal to scientific men universally as an accurate standard. Camlles have been used for a number of years as standards in spite of their many sho•tcoinings: and we are now accustomed to have our electric and other lights rated in candle power. The candle in use in the United States and England for photometric tests is the stand ard spermaceti camlle, which burns I20 grains of Material per hour and six of which weigh one When this is prepared according to (.er
taiu as to size and form of wick, and burns with a Home 45 millimeters in height, the light emitted is considered unity. The stea rin star candle was used in France and a Sonic lint similar candle in Germany, while a paraf fin candle known as the l'crcinskerze ou aeconnt of its being adopted by of German Gas Alen was used in Germany and Au,trollun pry. The standard now commonly used in Ger many is the acetate of amyl lamp of Heffner, which can be so adjusted as to be equivalent to the English candle. In practical photometry secondary standards consisting of incandescent lamps which have been standardized by compari son with sonic fundamental standard are used, and ordinary lamps are tested by reference to them. l'alaz, /ndustrial Photometry, trans lated by Patterson (New York, 1894) ; Stine, Measurements (ib., 1900); Record of the Internotionol Congress of Physicists, 1900 (Park, 1901).