DETERMINATION OF HIGH TEMPERATURES BY REFRACTED RAYS. 461 



Report of the Committee, consisting of Dr. Crum Brown, Mr. J. 

 Dewar, Dr. Gladstone, Prof. A. W. Williamson, Sir W. Thom- 

 son, and Prof. Tait, appointed for the purpose of Determinating 

 High Temperatures by means of the Refrangibility of the Light 

 evolved by Fluid or Solid Substances. Drawn up by James De- 

 war, Reporter. 



It is well known that as the temperature of a solid is gradually increased, 

 the refrangibility of the emitted light increases likewise ; and as the result 

 we find red light emitted first, and gradually the other coloured rays appear 

 until we reach the ultra-violet rays. This correlation between refrangibility 

 and temperature was first experimentaUy proved by Draper* ; and it would 

 be a result of great importance to determine accurately the law of growth 

 of refrangibility with temperature. If this could be achieved, a very 

 easily applied and accurate pyrometer could be made of the ordinary spec- 

 troscope. 



There are various difiiculties, however, that beset this investigation at the 

 outset. First of all, the rapid gi'owth of the new rays confines the observa- 

 tions Avithin narrow limits of temperature ; secondly, the want of equal 

 sensibility of the eye for rays of all wave-lengths ; and, thirdly, the inter- 

 ference of diffused light preventing exact definition. It thus appears to be 

 futile to attempt or even expect accurate observations in these circumstances 

 through registration by the human eye, although, on first considering the 

 subject, it appears to be a very easy matter. Pinding no means of overcoming 

 these difficulties, unless by the use of complicated apparatus, involving the 

 use of rock-crystal prisms and lenses or fine gratings and the employment of 

 photographic registration, requiring time and thought previous to execution, 

 a series of observations have been made in the mean time on the increase of 

 radiation with temperature, an inquiry of vital importance with regard to 

 this subject. 



Becquerel, in his treatise on Light called ' La Lumiere,' has detailed a great 

 number of observations on the growth of luminous intensity with increasing 

 temperature. From these experiments he infers that " the difi'erences between 

 the logarithms of the luminous intensities are proportional to the differences 

 of temperature," proving that an exponential function of the form 



I=«(e*(T-e)_l), 



where I is the luminous intensity, T the temperature of the body, Q the tem- 

 perature at which the special ray begins to be evolved, a and h constants, and 

 c the base of the logarithms adopted. The values of a and 6, as deduced from 

 the experiment, for the red ray are respectively 0-00743 and 0-005014. The 

 above formula gives equally the growth of total luminous intensity if we take 

 as 500° C, that point at which the light-rays begin to be evolved, and a and 

 h as now having the respective values of 12053 and 0-00764. From the 

 last formula Becquerel gives the following values of the total luminous in- 

 tensity of a solid substance at different temperatures, stating it is probable 

 the above law does not hold above 1200° C. : — 



* Phil. Mag. 1847. 



