58 Messrs. Paterson and Dudding on Estimation of 



the colour-identity method. It is intended later to repeat 

 this experiment with other metals such as nickel, iridium, or 

 rhodium, using a neutral atmosphere to surround the incan- 

 descent wires. The difference of 1 per cent, in the tempe- 

 ratures given respectively by the carbon and the tungsten 

 colour standards must not be assigned too much weight. 

 Although the method is capable of a greater accuracy than 

 this, it is not claimed that the determinations described here 

 are correct to 1 per cent. 



Comparing again the usual optical methods and the colour- 

 identity method of estimating temperature, it is worth while 

 to see what is the explanation for the phenomenon which has 

 been described in this paper. The factor of chief interest is, 

 that if a black body at 1750° 0. radiates towards one side 

 of a photometer and platinum at the melting-point (1750° C.) 

 towards the other, there will be identity of hue on the two 

 sides of the photometer, even though the platinum is opera- 

 ting under open radiation conditions. The hue of the total 

 radiation of the platinum is, therefore, a measure of its true 

 temperature. 



If, on the other hand, a pyrometer be used to measure the 

 temperature, first of the black body and then of the melting 

 platinum, it will give a value of 1750° C. for the black body 

 and about 1550° C. for the platinum. The latter tempera- 

 ture will depend on the wave-length in which the measure- 

 ments are made, but whatever the wave-length used the 

 temperature given will be very much lower than the true 

 temperature of the platinum. 



Waidner and Burgess have given the melting-point tem- 

 peratures of platinum (black body) determined in three wave- 

 lengths as follows : — 



Colour. 

 Keel 



Table VI. 



Wave-length. 

 0-666/i 



Melting-point. 

 1,534° C. 



Green 



0-547/i 



1,578° C. 



Blue 



0-462/u 



1,610° C. 



Examining the cases of a grey body and then of platinum, 

 for which most data are available, it is possible to see from 

 what follows how closely the colour-identity method will 

 tend in practice to give the true temperature. 



Curve A (fig. 5*) shows the power distribution of a black 

 body at 1750° C. over the visible spectrum calculated from 



* Hyde, Journ. Franklin Inst. loc. cit. shows curves very similar to 

 these to illustrate points connected with his investigations on selectivity. 



