514 MR. J. E. PBTAVEL ON THE HEAT DISSIPATED BY 



In Table XVII. a few results obtained from the' readings of a thermopile, the 

 surface of which was coated with lamp black, are recorded. These results, though 

 much less accurate than those obtained by use of the bolometer, serve as a confirma- 

 tion of the previous values and show that these values are not affected by the type of 

 instrument used. 



PART III. ON THE VARIATION OF THE INTRINSIC BRILLIANCY OF PLATINUM 



WITH TEMPERATURE. 



The flux of light emitted by one square centimetre of platinum at a fixed 

 temperature has many times been proposed as a standard of light; By VIOLLE* 

 this temperature is defined as the point of solidification of the metal, whereas 

 LUMMEB and KuRLBAUMt advocate a lower temperature not far from the melting 

 point of palladium. It was essentially as a preliminary to the study of these two 

 standards that the present work was undertaken. The method employed for the 

 temperature measurements was in every particular the same as used in the first part 

 of this paper. The experiments were made with wires 1'12 millims. in diameter 

 enclosed in a gun-metal box 8 X 8'5 X 3 centims., which could be fitted with 

 diaphragms of any desired shape. The wire under observation was horizontal, the 

 points of contact of the potential leads being a little more than one centimetre to the 

 right and left of the edge of the diaphragm, the opening of which was 38 millims. in 

 length. The walls of the gun-metal casting forming the enclosure were hollow, and 

 a water circulation maintained the temperature at about 15C. 



As standards of comparison, incandescent lamps were used. These were first left 

 burning for from fifty to one hundred hours to avoid the initial rapid change in 

 candle-power and then compared with a number of standard candles ; the usual 

 corrections being in all cases applied where the candles burnt more or less than the 

 prescribed forty grains of sperm in ten minutes. The candle-power of these lamps 

 was finally established from the mean of some hundreds of photometric readings. 

 While any one of these lamps was in use, the electromotive force at its terminals 

 was kept constant by the aid of a potentiometer. No photometric reading during 

 which the electromotive force had varied more than '05 per cent, from its correct 

 value, was recorded. 



Where the lights differ largely in colour, as was the case in most of the observa- 

 tions recorded in the table, the measurements become influenced mainly by two 

 physiological phenomena 



(1.) The relative value of two lights of different colour depends on the absolute 

 illumination of the two surfaces used in comparing them. 



* ' Comptes Rendus,' 1884, p. 1032, vol. 98. 

 t ' Elektrotechnische Zeitschrift,' 1894, p. 475. 



