262 On the Absolute Wave-length of Light. 



It is possible, however, to check this result by referring 

 S a 2 to the Berlin platinum standard through the medium of 

 the Coast Survey metre "No. 49." This latter standard was 

 compared in 1876 with metre 1605 and directly with the 

 platinum metre. The details are given in Prof. Foerster's 

 report contained in the Eeport of the Coast Survey for 1876. 

 The result of the direct comparison was 



Pl-"49"=+24'4/ i . 



But now Prof. Rogers has compared R 2 with " 49," ob- 

 taining in terms of the assumed length of R 2 



"49"=A-19-3yu; 



the assumed value of R 2 was 



R2=A + l-3/*. 

 Hence we have 



R 2 -"49" = 20'6/*, 

 from which follows 



PI -R 2 = 3-8/*. 



If now the equation between PI and the Metre des Archives 

 established by direct comparison in 1860 be correct, 



A -P1= -3-01/1. 

 And therefore 



R 2 -A = -0-8/4, 



a result which is in close accordance with those derived from 

 the Conservatoire metre and Type I. of the International 

 Bureau by means of the Standards T. and C.S. 



In my final determination of wave-length, I have used the 

 mean value of S a 2 as derived by the foregoing methods. 

 Collecting equations, 



S a 2 + (f-96 = £A„. FromT. 



S a 2 + r-04=£A . „ c.s. 



S a 2 + F-40 = iA . „ "49." 



S a 2 -P-68 = JA . » R 78- 

 Giving to the equations derived from C.S. and Rfs twice the 

 weight of the others, we have finally, 



I have given the relations derived from C.S. and R^ double 

 weight because these standards have been compared directly 

 with the standard of the International Bureau, which now, 

 probably, should be regarded as the ultimate standard of 



