470 



Mr. J. E. Petavel. An Experimental 



quantity of water vapour in litres per cubic metre of air, the light L 

 sent out will vary according to the formula — 



L - 1-049 -0-0055,' . 



This will cause a variation of about 4 per cent, from one season of 

 the year to another. The variations due to this cause are stated to be 

 still more marked in the Har court and Carcel lamps. 



Again, if X\ represent the quantity of carbon dioxide present in the 

 atmosphere, measured in litres per cubic metre, the light will be given 

 by the formula — 



L = 1-012-0-0072^. 



Variations in the height of the flame are of the greatest importance. 

 If h is the height — 



L = 1+0-025 (A -40), 

 or L = 1 - 0-030 (40 - h), 



according as h is above or below 40 mm.* The mean variation is, 

 therefore, over 2J per cent, per mm. Owing to the bright halo which 

 surrounds the flame, it is by no means easy to adjust the height 

 correctly. 



Finally, although it was at first stated that the degree of purity of 

 the amyl acetate had no very considerable influence on the light, this 

 has of late been denied, some authorities going so far as to state that 

 sufficiently pure amyl acetate cannot be obtained in France.! 



These facts will suffice to show that the variations are mainly due to 

 causes inherent in this class of standard. Some of the difficulties can 

 be obviated by providing a chemically pure atmosphere, and researches 

 are being carried out in America in this direction, but it is obvious 

 that any such improvement will involve a considerable complication of 

 the apparatus. 



Incandescent Standards. 



In the case of standards of this class, the constancy of the light 

 depends essentially on the constancy of the temperature at which the 

 radiating body is kept, and on the constancy of the emissivity of the 

 body at that temperature. 



The temperature may be fixed by some definite physical phenomena 

 as in the Blondel and Violle standards, or it may be determined in a 

 more or less arbitrary manner, as in the Lummer and Kurlbaum 

 standard. 



* ' Journ. fur Gasbeleuchtung und Wasserversorgung,' vol. 31, p. 583, 1888, or 

 ' Elektrotectmische Zeitschrift,' vol. 9, p. 96, 1888. 



f ' Rapport sur les Unites Photometriques,' A. Blondel, Congres International 

 des Electriciens, Geneve, 1896. 



