[ 314 ] 

 XL III. Intelligence and Miscellaneous Articles. 



A SPECTROMETRY STUDY OF SOME SOURCES OF LIGHT. 

 BY A. CROVA. 

 r |lHE general law of emission of the radiations emitted by a body 

 *~ raised to an elevated temperature is not completely known. 

 Dulong and Petit * have given the empirical law of the obscure ra- 

 diations emanating from a body heated to temperatures below 240°; 

 and Edin. Becquerelt has demonstrated that the intensity of the 

 red, green, and blue radiations varies with the temperature of the 

 body which emits them, according to an exponential law analogous 

 to that of Dulong and Petit. 



The exponentials which represent the law of emission of radia- 

 tions of different refraugibiliti.es are represented by curves of which 

 the origin corresponds to the temperature at which the radiation 

 considered commences to be produced, and rises the more rapidly 

 as the wave-lengths of the radiations become less. According to 

 M. Edm. Becquerel, the logarithms of the bases of these exponen- 

 tials vary in the inverse ratio of the wave-lengths of the radiations. 



These considerations may serve as a starting-point to a method 

 of determination, in a spectrometry way, of the temperature of 

 incandescent solids or liquids. In fact it follows from the investi- 

 gations of Mr. Draper J and M. Edm. Becquerel that, when the tem- 

 perature of an incandescent solid increases in a continuous manner, 

 the spectrum of the radiations emitted by it lengthens towards the 

 violet end, and that each of the radiations of this spectrum is at the 

 same time increased in intensity according to an exponential for- 

 mula. The temperature of the luminous source can therefore be 

 measured : — (1) by means of the wave-length of the radiation which 

 limits the spectrum towards the violet ; (2) by the position of the 

 thermal maximum of the spectrum, which approaches nearer to the 

 violet in proportion as the emission-temperature becomes higher ; 

 (3) by means of the ratio of the luminous intensity of a determi- 

 nate radiation A, taken in the spectrum of the source, to the inten- 

 sity of the same radiation in the spectrum of a source of known 

 temperature, compared with the ratio of the luminous intensities of 

 another radiation A' in the same two spectra. 



These last determinations can be easily effected by means of a 

 spectrophotometer. Several observers have made use of instru- 

 ments of this kind §. I used that of M. Glahn, w 7 hich permits 

 measurements to be made upon homogeneous radiations. 



On the other hand, I have measured the thermal intensity of the 

 simple radiations of the solar spectrum by means of a linear ther- 

 moelectric pile and a very sensitive galvanometer, using for the 



* Ann. de Cfiimie et de Physique, 2 e serie, t. vii. 



t Edm. Becquerel, La Lumiere, t. i. pp. 61-67. 



t Phil. Mag. 1847, vol. xxx. p. 345. 



§ Govi, Comptes Rend us, t. 1. p. 156 (1860). Trannin, Journal de 

 Physique, t. v. p. 207. Vierordt, Pogg, Ann. 5th series, vol. xx. Glahn, 

 Wiedemann's Annalcn, vol. i. (1877)." 



