REVERSED AND NON-REVERSED SPECTRA. 



131 



the data for /IQ i. For the case of the F lines, the B values found by com- 

 parison with the standard juo i (last column of table 12) might be taken as 

 correct within the error of method. Nothing, however, has been found to 

 account for the correspondingly large values of BO for the C line. 



75. Distortion of glass absent. To test the effect of possible distortion of 

 the end plates of the tube, a shallow cell was constructed but 0.8 cm. deep, 

 closed by plates of the same glass. The diameter of the tube was identical 

 with that of the long refracting tubes. Tests made with the electric arc and 

 sodium line gave the mean values 



Plenum io 3 A7V=5.47; 6.6 cm. 



Vacuum 5-25; 6.4 cm. 



Thus the effect of exhaustion is 0.0002 1 cm. The long tube gave, on the aver- 

 age, 0.039 cm. for 138 cm. of length. Hence the air effect should be 



0.039X0.8/138 = 0.00022 



which is practically identical with the value found. Hence there is no per- 

 ceptible distortion referable to the glass plates. 



TABLE 13. Dispersion of air. D and F lines. Single Tube, length 138 cm. Barom., 75.40 

 at 28. =74 cm. Temp. 20. St. refers to standard data. A = ^ i. 



76. Further observations with sunlight. In the absence of other than 

 inferential reasons to account for the difficulties met with, a final series of 

 observations was made between the D and F lines and a single tube, with 

 the results given in table 13. The mean value of B found directly, viz, 1.70 

 Xicr 14 , would be admissible; but the corresponding values of Mo i as com- 

 pared with the standard values are again too large and worse than above. 

 The same is true of the values of B found by comparison of AN/e with the 

 standard values of (JL O i, and their coefficients come out differently for the 

 C and F lines. In fact, the discrepancy of MO i is now about 3 per cent, 

 whereas observations for AN/e should not be in error more than (2 X 1/400 = 

 0.0025) 0.5 per cent. There is thus something variable at the limit of appli- 

 cation of the present method which has persistently escaped detection. I 

 have thought that a distortion associated with the form of the interference 

 pattern in passing from C to the F line may be in question, as the discrepancy 



