244 Dr- Miillev on the Thermal Effects 



another, as is seen in fig. 1, p. 342, in which the curve R a c S 

 belongs to the spectrum of the rock-salt prism. 



In this spectrum accordingly, the thermal maximum lies still 

 further from the red than in the spectrum of the glass prism. 

 And indeed (agreeably also to Melloni's results) the distance of 

 this maximum from the boundary of the red is about as great as 

 that of the transition of the green into blue from the red boun- 

 dary of the specti'um. 



The dark thermal extension of the spectrum, according to 

 these experiments, is not greater for rock-salt than for glass. 



As rock-salt allows all kinds of thermal rays to pass through 

 equally well, the curve R « c S represents the true distribution of 

 heat in a spectrum produced by refraction, and unmodified by 

 partial absorption. 



Estimation of the index of refraction, and of the undular length 

 of the extreme dark thermal rays of the solar spectrum. 



It has been established by the above experiments, that the 

 dark thermal rays contained in the solar spectrum extend far 

 beyond the red boundary of the visible spectrum, and that the 

 Fraunhofei-'s line B (for a crown-glass spectrum) lies about mid- 

 way between the violet end of the spectrum and its extreme dai'k 

 thermal rays (fig. 1, p. 242). Now, for crown-glass, the index 

 of refraction for H is about 1-546, and for B about 1-526 : hence 

 it follows that the refractive index of the extreme dark thermal 

 rays of the solar spectrum is about 1-506. 



It is clearly impossible to deterraiue the undular length of the 

 dark thermal rays of the solar spectrum, by employing the same 

 method as that used in finding the undular length of the 

 luminous rays of different colours. We must, on the contrary, 

 assume the undular lengths of the differently-coloured luminous 

 rays as known, and deduce the undular length of the extreme 

 dark thermal rays from their indices of refraction, by making use 

 of the subsisting relation between index of refraction and un- 

 dular length. 



The results of my experiments appear to disagree with Cauchy's 

 formula of dispersion, which is said to connect undular length 

 and refractive index. I purpose discussing this subject more 

 particularly on a subsequent occasion. 



I propose at present to endeavour to establish the connexion 

 between undular length and refractive index by means of an 

 empirical formula. Let us denote the undular length by w, the 

 refractive index by e, we may then put 



w — a + be + ce'^ (1) 



If, in this equation, wc put in succession three corresponding 



