REVERSED AND NON-REVERSED SPECTRA. 



129 



74. Differential and single refraction tubes. Sunlight. The direct experi- 

 ments for the coefficient B were now resumed and conducted with sunlight, 

 with the results given in table 1 1 . The first two series were made with the 

 two identical tubes specified, exhausted alternately, one tube containing a 

 plenum of air, while the other was nearly empty in each experiment. The C 

 and F lines alone were used for measurement. In spite of the large displace- 

 ment (AN = 0.07 6 to 0.078 cm.), the results were not as satisfactory as was 

 expected, owing to the fact that sharpness of vision is made difficult by the 

 stray reflected spectra to which reference has already been made. But the 

 data for B obtained with one exception (No. 2 in the first series) are consistent 

 and reasonably good. In every other respect the work was satisfactory and 

 could have been improved by using oblique cover-glasses. The values of B 

 obtained are therefore disconcerting. 



TABLE n. Dispersion of air. First and second series, Differential Tubes, each 138.0 cm. 

 long. C and F lines. Third and fourth series, Single Tube, good adjustment. 



I then went back to the single-tube experiments (in series 3 and 4), and 

 these are the smoothest results obtained. The C and F lines were used as 

 before. In the last series, for instance, the micrometer reading is the same 

 to io" 4 cm. throughout. In spite of this satisfactory behavior, the value of 

 B obtained is again of the same low order, all the data, both for double and 

 single tubes, being consistent throughout in this respect. Series 3 and 4 agree, 

 although p is changed from 74 cm. to 76 cm. of mercury. 



In table 12 I have summarized the data in comparison with the standard 

 results, computing /J.QI and B , for each of the cases, reducing all values to 

 oC. and 76 cm. of mercury. The difference of no i for the F and C lines, 

 which is 3.2 X io" 6 for the standard data, is but 2.3 X io~ 6 on the average in the 

 present results. Similarly, the mean values of the latter are 10^X4.1 and 

 10^X3.2 larger for the C and F lines, respectively, than the standard results. 



