REVERSED AND NON-REVERSED SPECTRA. 125 



ment. Disregarding earlier results, the following are mean values of the ten 

 independent data for AN (each comprising a reading for vacuum and for 

 plenum) : 



= 58.gcm. t = zz. 3 = 74.5011. 10^^ = 37.69 cm. /=i3S.ocm. 

 Thus 



where ^ refers to normal pressure and absolute temperature (r). If ju is 

 given for the D line, dp/ d\ is determinable. It will be sufficient for the present 

 purposes to put ^ = .4+5/X 2 , or \.dn/d\= 2.5/X 2 



5 referring to r and . Mascart's * value for /z i (agreeing with Fabry's) 

 is 10^X292.7, whence 



2?=io- 14 Xi.34 at r and p 



If the value B be computed from Mascart's observations between C and 

 , D and F, respectively, 



so that the mean value 10 "5=1.65 ma Y be taken. Since the last decimals 

 of M are in question, it will not be correct to more than 5 to 10 per cent. 



The value found above (io 14 2? = i.34) is therefore somewhat too small. 

 True, since from equation (3) 



(4) 



an error of lo" 4 cm. in AN is an error of 0.13 X icr 14 or 10 per cent in B. Very 

 close agreement can not therefore be expected in either result. One is tempted 

 to refer the present low value of B to flexure of the glass end plates of the 

 tube, which, when the tube is exhausted, become slightly saucer- shaped and 

 introduce a sharp concentric wedge of glass into the component beam, whereby 

 the interference pattern is changed, probably in the direction of smaller 

 values, as found. But the direct experiments below do not show this. In 

 any case, the measurement of B lies at the limits of the method. An advan- 

 tage may possibly be secured by using two identical tubes, one in each com- 

 ponent beam, the tubes to be exhausted alternately. The sensitiveness 

 would then be doubled. 



72. Observations with sunlight. Single tube. These observations are 

 given in table 9, the exhaustion throughout being 75 cm. and the temperature 

 about 1 6. In the first set sunlight was used without a condensing lens; in 



* See excellent summary in Landolt and Boernstein's Tables, 1905, p. 214. 



