Dimensions of a Body on its Thermal Emission. 271 

 whence tbe emissivity (e) becomes 



7 c 



a (log R — log a) 1 



or for convenience of calculation, 



It h 



«(logi R— log 10 a)' 



This, then, is presumably an approximation to a theoretic 

 formula for the case of experiments like those of Messrs. 

 Ayrton and Kilgour referred to before ; and the accompanying 

 Table 1. and curves (fig. 1) (pp. 272 & 273) show how closely, by 

 a suitable choice of constants, it can represent the experimental 

 values. The constants I determined by the method of least 

 squares ; this I considered it advisable to do, because the 

 experimental values are not sufficiently precise to enable one 

 to draw a smooth curve through them with anything like cer- 

 tainty. In particular I may mention that their own empirical 

 formulae fail to fit in with the experimental values yielded by 

 the wire of radius '0037; and these values are similarly shunned 

 by the formulae which I give. 



The first term in each of my formulae should represent the 

 radiation-constant. It must be noted, however, that for such 

 very thin wires radiation forms only a small portion of the 

 whole emission ; and therefore, without having results of 

 greater accuracy from which to deduce the formulae, it is not 

 possible to assign the value of this term with definiteness. Ail 

 the values for it are, however, higher than those obtained by 

 observers who have experimented on the emission from 

 surfaces in vacuo, as the following data show : — 



Radiation Values. 



/Bottoinley : Sooted globe in Sprengel 1 -000125 for excess of 83°-6. 



vacuum [-000095 „ ., 32°-7. 



from jBottomley: Silvered and highly po- 1 -0000354 „ „ 72°-l. 



Everett* Hahed globe j -0000296 „ „ 43°7. 



j Nichol (published by Tait) : Pressure 1 . nnnn ^ 7 AOO 



\ of air =10 mm. of mercury j UUUUD ' » » ** • 



As regards the other constant " b " ( = '4343 c), we have 



At 300° . . b = '0696xl0~ : \ c = '16 x 10~ 3 , 



200° . . 6 = -0620 xlO" 3 , c='143xl0- 3 , 



150° . . 6 = -0605 xl0~ 3 , 6- = -139xJ0- 3 , 



100° . . 6 = -0549xl0- 3 , e = '126xl0- 3 . 



