The construction of the instrument is such as to permit reversal by revolving the whole appa ratus upon a horizontal axis through 180 de grees. The face of the screen which previously was lighted from the source S by means of the mirror M and the prism ABG now receives its light from s and vice versa. Any lack of uniformity in the two sides of the apparatus is thus readily detected and can be eliminated.
Standards of Light.— When, owing to the development of the gas industry, the art of photometry began to take on commercial im portance, it became necessary to have recognized standards of comparison. The standard adopted in France was a vegetable oil lamp with me chanical draft burning colza-oil and known from its inventor as the Carrel lamp. The di mensions of the lamp were carefully specified together with the form and size of the chim ney and the amount of oil (42 grams an hour) to be consumed.
In England the light unit adopted by gas manufacturers and subsequently legalized by the board of trade was the light given by a candle. The British standard candle was made of spermaceti. It was slightly conical in shapd for convenience in molding and of such size (.f inch in diameter at the bottom, 10 inches long and inch in diameter at the top) as to consume 120 grains of wax per hour. The wicks were made of three strands of cotton, each strand consisting of 18 strands.
In Germany the manufacturers of illuminat ing gas adopted as their standard a candle of paraffin. This candle, which is known as Vereinskerze had a diameter of 22 millimeters. The wick consisted of 25 strands of cotton. The height of the flame when burning normally was 50 millimeters. In Munich the legalized standard agreed upon between the city and the gas company was the light from a stearine candle slightly conical in form, of a mean diameter of 20.5 millimeters, the height of the flame being 56 millimeters, The practical advantage of the candle as a light unit, which consists in the fact that it is a form of light with which the public is familiar, led to the introduction in France of a standard candle known as la bougie de retoile. This was a stearine candle consuming 10 grams an hour. Its light was approximately equal to + of a cartel. The height of the flame was 52.5 milli meters.
Extended studies of the performance of standard candles and countless attempts to de termine the relative intensities of the various forms legalized in the different countries have only served to demonstrate the inadequacy of this source of light even when prepared with the utmost care, to serve as a standard in pho tometry. Almost the only advantage such a
unit possesses is found in the fact that the term has a familiar sound to the public.
The range and character of the fluctuations of the British standard candle may be seen from the curve in Fig. 7 which is taken from measure ments made by Sharp and Turnbull ((Trans actions of the American Institute of Electrical Engineers,) Vol. 13, pages 145, .1896). The curve covers a period of 60 minutes during which the brightness of the candle was observed at intervals of 30 seconds. It will be seen that the intensity of the flame is subject to frequent fluctuations often amounting to more than 10 per cent.
The altogether unsatisfactory performance of the various forms of standard candle has led to numerous attempts on the part of pho tometrists to find a more reliable standard of light. A form of standard lamp introduced by Methven (1878) and consisting of an Argand Extensive experiments with lamps burning vegetable and animal oils and with a variety of petroleum lamps have only demonstrated the necessity of a proper fuel.
One of the few available fuels of known chemical composition is amyl acetate. A stand ard lamp in which this liquid is used as the combustible was described by Hefner-Alteneck burner for ordinary illuminating gas with a metal screen which cuts off all but the central portion of the flame has been extensively used in gas photometry. The tests of this standard have shown, however, that it is subject to vari ations but little less than those of the candle, owing to the variable character of the gas em ployed. In 1877 Harcourt introduced a standard lamp in which the petroleum product known as pentane is the fuel employed. This substance which is obtained by fractional distillation can not be readily obtained in complete purity, but the distillate which boils off at C. consists largely of pentane with a small proportion of other closely allied hydrocarbons. The liquid is highly volatile and in the Harcourt lamp the vapor is ignited at the end of a metal tube extending about two inches above the wick. A modified form of the pentane stand ard lamp is extensively used in gas photom etry in the United States and is said to give excellent results.