Complete Emission Function. 383 



low temperatures and for long-period waves at all tem- 

 peratures. Paschen (/. c.) has noted the possibility of even 

 such partial radiators as glass radiating as a black body at 

 very low temperatures. This class of maxima is associated 

 with the so-culled continuous spectra. 



II. A second class of maxima vary only slightly in position 

 as the temperature is changed. These maxima are low and 

 broad, but yet sharper and higher than the maxima in the 

 spectra of perfect radiators at the same temperature. Such 

 maxima we find commonly in the infra-red spectra of the 

 crystalline salts, glass, water-vapour, and the like. The 

 variation in the position of the emission maximum in the 

 carbon-dioxide spectrum has been studied by Paschen"*. He 

 found the wave-length of the maximum varied about 3 per 

 cent, as the temperature was raised from about 600° to 1700° 

 absolute. "We have not yet been able to study the emission 

 maxima of the crystalline salts; but the work of Abramczyk f 

 on the radiation from quartz indicates that we might expect 

 such a variation in the maximum. The Kirchhoff law should 

 hold in this region for this radiation, and if a plate of quartz 

 does not absorb all the radiation emitted by a thinner plate at 

 a higher temperature — as observed by Abramczyk — may we 

 not most easily account for the fact by supposing that the 

 positions of the emission and absorption maxima were different 

 at the different temperatures? Whatever it is, the law of the 

 variation is probably not a simple one ; and this quite agrees 

 with the mathematical expression developed later on. 



Til. A third class of emission maxima do not vary at all in 

 position with the temperature, though in the regions of tem- 

 perature and period in which they occur the isochromatic 

 emission temperature function is probably the same as that 

 which holds for perfect radiators. These maxima are, as a 

 rule, narrow and sharp, and the emission in the region in 

 which they occur is practically all confined to the maxima 

 themselves. The emission at the maxima may be regarded as 

 infinite in comparison with the emission at a short distance on 

 either side of the maxima. This class of maxima (spectrum 

 lines) are associated with high temperatures and short periods. 

 They probably never occur in the same period-region with 

 maxima of class I., at least when the radiator is an element or 

 a chemically simple compound and when no chemical change 

 occurs. We cannot draw such a distinction between maxima 

 of types II. and III., nor is such a distinction drawn mathe- 

 matically in the complete function. 



* F. Paschen, WiecL Ann. 1. p. 440 (1893). 

 f Abramczyk, Wied. Ann. lxiv. p. 647 (1898). 



