Estimation of High Temperatures by Colour Identity. 35 



The work described in this paper is not in the nature of a 

 complete investigation of the subject. It had for its original 

 object the determination of the colour of the light from 

 molten platinum under the open radiation conditions which 

 prevail in the realization of the Violle standard o£ light. A 

 good primary standard of light must not only be constant 

 and accurately reproducible, but the colour of its light should 

 approximate to that of the sources which are ordinarily used 

 in practice, so that large colour differences will not be in- 

 volved in the photometric measurements for which such a 

 standard is used. The object for which the investigation 

 was started was completed over two years ago, but the pro- 

 gress of the work indicated some unexpected phenomena 

 which it was intended to investigate further. Pressure of 

 other work has up to now prevented this being done, and 

 the authors desire at this stage to publish this preliminary 

 note on the subject. The accuracy of the work is the accuracy 

 of preliminary experiments in which all reasonable precautions 

 have been taken. Values given for tomperature certainly 

 have not an absolute accuracy of more than 1 or 2 per cent., 

 but the methods described are capable of a higher precision, 

 and this will undoubtedly be attained in the fuller investigation 

 which it is intended to undertake. 



General Discussion. 



Optical pyrometry is almost exclusively concerned with 

 the intensity of the li^ht emitted by a luminous body in any 

 given wave-length. The colour of the light thus dealt with 

 is fixed by the wave-length or wave-lengths chosen for the 

 measurements, and colour differences do not occur. 



In ordinary photometry the sum of the intensities of the 

 light emitted by a source in all wave-lengths over the visible 

 spectrum is compared against the sum of the intensities of 

 the light emitted by another source. The radiation from 

 each source has thus a composite colour whose characteristics 

 will depend on the relative intensity of the light in each 

 wave-length. Although both sources may radiate according 

 to the law of a black body, if there should be a difference of 

 temperature between them the composite colour or hue of the 

 radiations from the two bodies will differ, and it becomes 

 necessary to compare intensities which are not of the same 

 colour. For most solid radiators the colour of the light is a 

 perfectly definite quality, and forms a criterion of the state 

 of incandescence of such bodies. Most bodies are more or 

 less selective in their radiation, but there is a certain group, 

 -consisting mainly of metallic substances, which although 



D 2 



