148 THE INTERFEROMETRY OF 



vanishing when a plenum of air and the adiabatically exhausted air, respec- 

 tively, are introduced into one of the beams. Since 



/4) I = Ho\/2e = 



this is equivalent to 



90. Experiments with long tubes. Diameter, one inch. The difficulty 

 encountered in the case of the preceding experiments was the small value of 

 the displacement AN obtained. As a consequence, every little incidental 

 disturbance produced a large effect in 7. It is the purpose of the present 

 experiments to remedy this defect by using long tubes by which AN" may be 

 increased over ten times. It was particularly of interest, moreover, to begin 

 with relatively thin tubes, and inch gas-pipe suggested itself for the purpose. 

 The value of 7 to be expected will necessarily be too small, as the air must 

 undergo reheating before the exhaust cock can be closed. The question, how- 

 ever, is whether consistent values of 7 will be found, even for these extreme 

 conditions and for large variations of pressure. Obviously the window plates 

 will not produce discrepancies, as has been directly shown in paragraph 88. 



The gas-pipe installed was 143.4 cm. long within. To make the junction 

 with the vacuum chamber, a straight pipe of the same diameter and about 

 75 cm. long was needed between the main pipe and the 2^-inch stopcock. 

 The connecting pipe, together with the tube itself, is probably the chief cause 

 of the resistance to flow and the low value of 7 found, but it was not possible 

 to shorten it. 



The large stopcock inevitably leaked slightly when the pressures were 

 different in the two chambers; but immediately after exhaustion this made 

 no appreciable difference, as the two pressures are then nearly the same. In 

 fact, no rings vanish from the spectrum from this cause. Just before exhaus- 

 tion, however, after closing the gas-pipe by the fine influx stopcock, appreciable 

 leakage is shown by the spectrum. Hence the exhaustion must be made 

 immediately after the influx cock is closed. Some low results at the outset 

 are referable to this difficulty. 



The tube was, as usual, filled with dry air after exhaustion. The results 

 are given in table 18, in the same way as in the preceding case. The experi- 

 ments themselves were throughout satisfactory, no difficulty being encountered 

 at the interferometer. The work, moreover, is equally trustworthy at low 

 and at high exhaustions, a result which is rather surprising. In the latter 

 case, as the total displacement, AN, is over 0.0276 cm., the 7 contained should 

 be correct within i per cent. 



Only one attempt was made to find AN by the march of the interference 

 fringes. Fully 276 were observed, and it is here necessary to count the fringes 

 passing the D line, since the ellipses are displaced throughout the greater part 

 of the length of the spectrum ; but this introduces no inconvenience whatever. 

 The difficulty is due to the time needed in counting so many evanescences; 

 for during this interval the electric lamp is liable to flicker seriously, or some 



