REVERSED AND NON-REVERSED SPECTRA. 



151 



would be so rapid. It would seem to be probable, therefore, that if a 4-inch 

 tube were used the conditions for obtaining a trustworthy value of 7 would 

 be nearly met. 



As the exhaustions in a successive series are gradually increased (initial 

 partial vacua from = 56.46 cm. to = 20.36 cm. in the vacuum chamber), 

 the observed values of 7 gradually but slowly decrease, the mean values being 

 Oo = 75-7 cm. to 75.9 cm.) 



= 56.46 

 7= i-3 2 



38.36 

 1.29 



29.36 

 1.25 



20.36 cm. 

 1.26 



where the fourth value is too small, for incidental reasons. This general 

 result is also to be expected ; but it is rather remarkable that with such high 

 exhaustions as those finally used the decrease of 7 is not more marked. 



The work, as a whole, progressed smoothly throughout, the only interfer- 

 ence with precision being the incidental occurrence of open ellipses. To obtain 

 other patterns would have required longer additional adjustment than the 

 work at the present stage seemed to warrant. 



92. The same. Diameter of tube, four inches. The first experiments made 

 with the 4-inch tube are given in table 20. The completed apparatus showed 

 a slight leak, which could not be detected after long searching. The tube 

 was therefore admitted for a tentative series of experiments. The exhaust 

 pipe here, as above, was rigid and straight, but only 2 inches in diameter, 

 with a 2^-inch stopcock. To exhaust the air-chamber, the handle of the 

 cock was suddenly jerked over an angle 180 between the two closed positions. 

 The plug virtually floated in oil, as shown elsewhere. 



TABLE 20. Values of -y. Brass pipe, 4 inches internal diameter. 

 C=952. 6; i+x= 1.0341; e= 126.9. (V+v)/V= 1.119. Small leak in apparatus. 



As a whole, the results are disappointing; and they are irregular, for mean 

 readings could not be made because of the leak. They are, nevertheless, 

 interesting, inasmuch as with some of the above data they point out a special 

 source of discrepancy. It will be seen that the 7 values tend to decrease in 

 successive measurements, beginning with a high value, which is here nearly 



