38 Messrs. Paterson and Duddino- on Estimation of 



Section 4 deals with the colour of the radiation from 

 molten platinum in relation to the practical usefulness of the 

 Violle standard of light. 



1. Electric Sub- standards of Colour for the Determination 

 of Temperature. 



In spite of the fact that the device of colour comparison by- 

 means of a Lummer-Brodhun photometer hns been used for 

 many years by various observers for obtaining equality of 

 efficiency of glow-lamps of the same type, it is not generally 

 realized how easily and with what precision such colour 

 comparisons can be made. Morris*, Stroud, and Ellis 

 employed this method in 1907, and extensive use has been 

 made of: it for investigating selectivity and other properties 

 of radiating substances by E. P. Hydef, with whom Cady 

 and MiddlekaufT have sometimes collaborated. Hydet, in 

 discussing the question of colour identity and temperature 

 (p. 40, loc. cit.), showed that a colour match with a black 

 body might be regarded as indicating that the temperature 

 of the black body was at least as high or higher than that 

 of the body compared against it, but he expressed the opinion 

 (p. 39) that under the condition of colour identity two 

 different radiators although with continuous spectra would 

 not be at the same temperature. 



A black body furnace electrically heated and capable of 

 being raised to a temperature of 2200° C, with a clear in- 

 ternal atmosphere, was kindly put at the disposal of the 

 authors by Dr. J. A. Harker, F.R.S. The very excellent 

 arrangements of this furnace need not be explained in detail 

 here. The apparatus is shown diagrammatically in fig. 1. 

 A is a black body kept clear of fumes by a stream of nitrogen 

 admitted at K. B is a diaphragm with glass window, which 

 permits only light from the centre of the incandescent surface 

 to pass down the photometer bench, and J are screens to cut 

 off extraneous light. G is a carefully seasoned electric lamp, 

 the current through whose filament can be accurately 

 measured by means of ammeter D. Between the two at E 



* Morris, Stroud, and Ellis, ' Electrician,' vol. lix. p. 584. 



t Hvde, Cadv, and Middlekauft, " Selective Emission of Incandescent 

 Lamps'," 111. Eng. Soc, New York, vol. iv. 1909, p. 334. Hyde, " Physical 

 Characteristics of Luminous Sources," Lectures, Johns IIopkin3 Univer- 

 sity, 1910. Hyde, " Radiation Laws for Metals," Astrophys. Journ. 

 vol. xxxvi. 1912, p. 89. 



X Hvde, "The Physical Production ot Light/' Journ. Franklin lust, 

 vol. clxx. 1910. 



