775 Prof. H. Rubens and Mr. H. HolWel 



Hence the value of the 



wave-leugths from the 



following 



computation : — 













Series a. 







Series /3. 





a-e =36*5 







*~f 



= 45-7 





a'-d' = 2S'l 







a f -e' 



= 37-1 





b-d =18-0 







b-e 



= 27-3 





b'-c'= 9-6 







b'-d' 



= 17-9 





92-2 









128-0 





09.9 

 X =~- • 10-46 = 96-5 y 



a. \ — 



128-0 

 14 ' 



10-46 = 



95-6 p. 





Series 7. 











«- 



-?='54"9 











h- 



-/=36'6 











c • 



-e =18*0 











a>- 



V=55*2 











//- 



-f = 36-3 











/ 



6* - 



V=17-9 

 218-9 









21 y 



1 









/V — 



24 



. 10-46 = 



95-4/^ 







Series four and five s 



>-ave X = 



97-8 a 



and A 



o = 96-9 it 



respectively. 



The logarithmic decrement computed for the third series 

 was 7=0'23, whereas the series a gave a value somewhat 

 smaller than 0*23 and /9 somewhat larger. The curves plotted 

 in figs. 11 and 6 a are based upon the value 7 = 0'23. 



Comparison with Former Wave-length Determinations. 



As has been mentioned at numerous times, the mean 

 wave-length and the energy distribution are dependent, not 

 alone on the material of the mirrors, which selectively reflect 

 the rays, but on the number of such plates, the angle of 

 incidence of the impinging ray, the distribution of energy in 

 the source of radiation, the absorption of the measuring 

 instrument, and the selective absorption of all media which 

 the optical path includes. Compared with the effect due to 

 the selective reflexion of the various plates, the other cir- 

 cumstances are all of minor influence, especially w T hen the 

 number of plates is not too small. 



In previous wave-length measurements with the grating 

 it was not possible to separate the two bands because of the 

 immense width of the slit which had to be used in con- 

 sequence of the limited energy. As a mean wave-length for 

 Reststrahlen of rock-salt and sylvine the values 51 '2 /j, and 

 61*1 fju were obtained *. The mean values, as obtained from 



* H. Rubens and E. Aschkinass, loc. cit. pp. 246, 247. See also E. F. 

 Nichols and W. S. Day, Phys. Rev xxvii. 1908, p. 225. These observers 

 obtain 52'3/i for the rnean wave-length of the Reststrahlsn of rock-salt. 



