150 



THE INTERFEROMETRY OF 



91. The same. Diameter of tube, two inches. The experiments were now 

 continued by enlarging the diameter of the tube to 2 inches. Brass gas-pipe, 

 1.35 cm. long, to be closed with thick glass plates, was at hand. To connect 

 the same with the vacuum chamber, a similar 2-inch pipe, 115 cm. long, as far 

 as the 2^2 -inch stopcock, was necessary. Moreover, as this was in the way of 

 the light received from the grating, the beam was reflected by an offset con- 

 sisting of two silver mirrors in parallel. No difficulty was found with this 

 arrangement, and the sodium line was in view to give evidence if any acci- 

 dental displacement should occur. 



Unfortunately, the ellipses obtained were somewhat irregular open forms 

 (i.e., half ellipses), and the endeavor to secure small closed patterns did not 

 succeed. This annoyance depending chiefly on the parts of the mirror and 

 grating used, and on shifting accessories, is not easily controlled. The indi- 

 vidual measurements of AAf are therefore not as good as those recorded in 

 table 1 8, where a displacement of icr 4 cm. was assured. They suffice, how- 

 ever, for the present purposes. 



The new data are given in table 19, t being the temperature of both cham- 

 bers, po the initial normal pressure of the air-chamber (2-inch pipe), and p 

 that of the vacuum chamber. 



TABLE 19. Values of 7. Brass gas-pipe, 2 inches internal diameter. 

 C= 952.6 ; i+x= 1.0341. 6=135.3 cm. (7+)/=i.049. 



The effective value of 7 in these experiments is, for the lower exhaustions, 

 above 7 = 1.3, showing a considerable improvement over the data for the inch 

 tube, which were not much above 7 = 1.1. This was to be inferred, of course; 

 but it was not expected that the increment of 7 due to increased diameter 



