Measurements in the Extreme Infra-red Spectrum. 771 

 And as a mean, therefore, f 



7i = 0-074, 

 and for the corresponding y 2 we have 



W=7ir =7i -"t, =0-084. 

 A i n — l 



The logarithmic decrement, as obtained from our curve, is 

 always somewhat larger than the energy distribution of the 

 rays requires. This is dependent upon the divergence of 

 the rays which pass through the air-layer. From the centre 

 of the air-film, where the image of the luminous mantle is 

 located, the rays diverge to the rim of the concave mirror Gr 

 (Hg. 2), so that the extreme rays form an angle of 2'h degrees 

 with the central ray. Since the cosine of this angle differs 

 from unity by only 001, the error introduced by over- 

 looking such a correction is negligible. 



There remain, however, the constants <f>\ and </> 2 to be 

 determined ; these are the corresponding values of the inten- 

 sities of the two bands. As it is immaterial in what units 

 the energy curve is plotted, the ratio of these two quantities 

 only is of importance. By a comparison of the maxima and 

 minima at those points where the interference fringes are 

 most and least pronounced, this ratio is procured. In the 

 first case the maxima are additive, while in the second they 

 are subtractive, so that if we again choose h'g. 4 as an 

 example, it follows that 



4>-2 _ v /'„„' . hi' K — h P r> 



01 \ /»art' • ki'K + /< e e' 



<£•: _ V /'-\- ■ /'.v.v' — hn'o 

 01 \/lliK • Kb' + hn'o 



respectively 



Substituting the respective values of h lia \ h? K , h ss > given 

 above, as well as /i ie ' = 0*9 mm., and /*»' o =0'6 mm. in these 

 equations, the values 0*65 and O'bO are obtained respectively, 

 in the mean 0*625. 



The energy distribution curves of the Reststrahlen of 

 rock-salt, sylvine, potassium bromide, and potassium iodide 

 were drawn according to formula (1) with the constants Xi, 

 A21 yu Y-2-> iir| d 02/01 thus calculated. They are shown in 

 PI. XIII. figs. 8, 9, 10, and 11. That these are approximately 

 correct representations of the energy distribution of the 



3 D2 



