A. MATHEMATICS AND ASTRONOMY. 45 



radiation of heat with the help of his actinometer, which 

 same instrument he also employed for the measurement of 

 the radiation of heat from the sun. It is known that the 

 temperature of the plate of zircon could not be much higher 

 than 2500, which is the temperature of the flame in which 

 it was heated ; but Soret finds that, if he applies to his ob- 

 servations with the actinometer the principle applied by 

 Secchi, he receives for the resulting temperature of the plate 

 of zircon a value of 46,000 ; whence it must be concluded 

 that the law of proportionality between the radiation and the 

 absolute temperature of a body is extremely erroneous al- 

 ways giving results perhaps twenty times too great. 7 C^ 

 1873, 411. 



THE SPECTKA OF THE SUN's LIMB AND CENTEE. 



Mr. Hastings, of the Sheffield Scientific School of New 

 Haven, has endeavored to overcome an inconvenience that 

 has hitherto impeded the direct comparison of the spectra of 

 the limb and centre of the sun. He has succeeded in bring- 

 ing the two spectra side by side in the field of view of his 

 spectroscope, by introducing in front of the slit a small prism 

 with four polished sides. The rays of light from the limb 

 of the sun, after entering this prism, are subject to a total 

 reflection, and while the telescope is directed to the centre 

 of the sun, so that its light falls directly npon the slit of the 

 spectroscope, the rays from the sun's limb, after passing 

 through the four-sided prism, also fill upon the slit, and the 

 observer receives them both simultaneously into his eye. 

 When the instrument is properly directed, and in adjust- 

 ment, we see a very narrow black line dividing the spectrum 

 longitudinally into two parts of widely diflferent intensity. 

 The fainter part, belonging to the limb of the sun, is marked 

 on its edge by the chromosphere lines. The difiei-ences be- 

 tween these two spectra are immediately recognized, and are 

 most pronounced when the sky is the clearest. Since the light 

 from the border of the sun undergoes a general absorption 

 within the solar atmosphere, so that its intensity is reduced 

 to much less than one fourth of that at the centre, Ave mio^ht 

 expect that the lines in the spectrum of the limb would very 

 materially difl*er from those seen in the spectrum of the cen- 

 tre of the sun. Hastings says that the. spectroscopic charac- 



