﻿200 M. F. Zollner on the Influence of Density and 



ference of temperature only. The following theorem may there- 

 fore be pronounced general : — 



When the difference between two spectra of one and the same in- 

 candescent gas is such that, for rays of any ref Tangibility, to a 

 maximum of the one spectrum a minimum of the other corresponds, 

 the temperatures proper to the two spectra must be different. 



The origination of spectra of different orders of one and the 

 same gas must therefore be regarded as a phenomenon evoked 

 especially by alterations of the temperature, and not of the den- 

 sity of the incandescent gas. 



8. In like manner as Kirchhoff empirically infers the conti- 

 nuity of the function J with alteration of the wave-length \ and 

 with constant temperature, the continuity of that function with 

 constant X and variable temperature may also be inferred. 



In reference to the first -mentioned continuity, Kirchhoff (/. c. 

 p. 293) remarks as follows : — 



" For a constant temperature the function J changes continu- 

 ously with the wave-length, as long as the latter falls short of 

 the value at which, for that temperature, J begins to vanish. 

 The correctness of this assertion may be inferred from the con- 

 tinuity of the spectrum of an incandescent platinum wire, as 

 soon as we assume that the absorptive power of this body is a 

 continuous function of the wave-length of the incident rays." 



This conclusion rests upon the fact that the spectrum of incan- 

 descent black opaque bodies presents to our eye the totality of all 

 the functional values of J proper to the different values of \ 

 in juxtaposition in space as they correspond to the temperature 

 of the incandescent body. Now, if the strength of the physiolo- 

 gical impression produced in our sensorium by the different 

 values of J in a spectrum were not likewise a function of the 

 wave-length, or, in other words, could we assume that the phy- 

 siologically determined ratio of intensity of two different parts 

 of the spectrum is equal to the mechanical, the course of the 

 function J with alteration of the wave-length could be repre- 

 sented simply by the photometrically determined curve of inten- 

 sity of a given spectrum at constant temperature of the incan- 

 descent body. 



Although, from the circumstance adduced, such a represen- 

 tation is not possible, and the nature of the physiological func- 

 tion according to which the optical impression of a homogeneous 

 ray changes with the wave-length while the vis viva is constant 

 is unknown, it may yet be maintained of this function also that it 

 must necessarily be continuous*; for if this were not the case, the 



* The notions " continuous " and " discontinuous " are not to be taken 

 here in a mathematical sense, but with reference to the absence or presence 

 of strongly prominent maxima or minima. 



