Physical Constitution of the Sun. 355 



made several experiments of this sort, and convinced myself of 

 the general practicability of the method*. It is besides known 

 to all observers of star-spectra, without the employment of pho- 

 tometric methods, that with the white stars in general the more 

 refrangible portions of the spectrum appear much more intense 

 than with the yellow and red stars. The temperature of the white 

 stars must, accordingly, be in general higher than that of the 

 yellow and red ones. When into connexion with this is brought 

 Father Secchi's remarkable observation, that the different types 

 of spectra are not distributed proportionately among the stars, 

 but in certain regions of the heavens the one or the other type 

 prevails, the inference is suggested that there are corresponding 

 differences in the stage of cooling of these provinces of our 

 fixed-star system, and that, if the time of origin of their conso- 

 lidated masses was the same, these differ in magnitude. 



§9. 



If the law of the emission of light found above as an ap- 

 proximation for incandescent platinum wires were, as to its form, 

 universal, and agreed with KirchhofFs function J for opaque 

 black bodies, it would necessarily be admissible to apply it to 

 heat-rays also. 



That between the heat-radiation of a body and its tempera- 

 ture there is no proportionality, I have already made clear in a 

 criticism on the method applied by Father Secchi to the deter- 

 mination of the temperature of the sunf. 



M. Soret has recently J proved, by interesting experiments, 

 that in fact the heat-radiation of a body increases much faster 

 than its temperature, and that consequently the hypothesis 

 made by Father Secchi in his actinometric determination of the 

 temperature of the sun, " the radiation of a body is proportional 

 to its temperature " §, was inadmissible. 



M. Soret put a zirconium plate into vivid incandescence by a 

 oxyhydrogen blowpipe, and determined the heat-radiation by 

 means of the actinometer which had served him for measuring 

 the solar radiation. On the hypothesis of the law of propor- 

 tionality applied by Father Secchi to the sun there resulted for 

 the temperature of the incandescent zirconium disk the value of 

 45,990° C. while in reality its temperature cannot have amounted 

 to more than 2500°. 



M. Soret's words (/. c. p. 228) relative to Father Secchi's 

 formula are the following : — 



* Vierordt has already made proposals for the photometric comparison 

 of star-spectra. Cf. Astron. Nachr. (1871) No. 1863, p. 237. 

 t Berichfe d. K. Sachs. Ges. 1871 (Feb. 11), p. 50. 

 X Archives de Geneve, vol. xliv. pp. 220-229 (1872). 

 § Le Soleil (Paris, 1870), p. 265. 



2B2 



