ISOCHROMATICS OF OLIGOCLASE. 1 29 



white light, although, as shown in curve c, fig. 66, the emissivity at 2 /1 

 is still weak when the temperature is close to the melting-point. 



In oligoclase it is not possible to locate so definitely, if at all, the position 

 of the maximum of the envelope by the intersection of the isochromatics; 

 for the lines show no tendency to curve, as in platinum, although at closely 

 the same temperature. By the extrapolation of the 2.9 p and the 4.4 p. 

 isochromatics, the intersection (obtained after correcting for slit-width) is 

 found to be at about 8.5 watts. The maximum emission would then lie 

 at about 3.5 /*, which is the position of the maximum of a complete radi- 

 ator at 850 abs. (580 C). On 29.4 watts, when the oligoclase was 

 already viscous, indicating a temperature of noo to 1200 C. (melting- 

 point 1300 ), the maximum emission would have to lie at a much smaller 

 wave-length than 3.5 ;i, say at 2 //, which is inconsistent with the observa- 

 tions that the emission curve is but little different in outline from those 

 obtained at lower temperatures, there being no indication of a shifting 

 of the energy distribution. It would therefore appear that it is not per- 

 missible to consider the envelope, drawn through the emission maxima, as 

 a criterion for judging the temperature of a substance like oligoclase. On 

 the whole, it appears that the general emission, as distinguished from the 

 bands of selective emission, is less intense in oligoclase than in most of the 

 other silicates studied. Such a substance, if it would withstand high tem- 

 peratures, would most nearly approach the ideal light producer. For, since 

 the absorption coefficient is small throughout the region, to 3 /*, where, in 

 a continuous spectrum, the greatest amount of energy is emitted, the emis- 

 sion spectrum would remain discontinuous, and only at the highest tem- 

 peratures would the general emission become of importance, when a large 

 amount of the energy radiated would be of wave-lengths affecting the eye. 



SUMMARY. 



In general, the results on the oxides furnish an excellent illustration 

 of the shifting of the maximum of intensity of emission toward the short 

 wave-lengths with rise in temperature, just as is known for solids emitting 

 continuous spectra. In this respect the various emission curves of zirco- 

 nium oxide are particularly conspicuous. In addition to what may be 

 termed " general emission," in which the maximum shifts with rise in 

 temperature, the curves of zirconium oxide are unique in having a sharp 

 band of " selective emission " which does not shift nor broaden with rise 

 in temperature. The results are not unlike those obtained by Anderson 1 

 for erbium oxide, in the visible spectrum. He found that the emission 

 spectrum was not continuous, but consisted of bright bands superposed 

 upon a continuous faint background. With rise in temperature the bands 

 became more hazy in outline, and at very high temperatures the spectrum 

 became continuous. If, according to Stark's theory, the continuous spec- 



1 Anderson: Astrophys. Jour , 26, p. 73, 1907. 



