128 



THE INTERFEROMETRY OF 



air supply A, from the tubulures e and e' of both refraction tubes T and T'. 

 These are therefore alternately exhausted. 



Preliminary results are given in table 10, the arc lamp with its sodium line 

 being used in the absence of sunlight. It will be seen that A2V, apart from 

 temperature (which is here higher than above), has been doubled. The 

 deflections were symmetrical within 0.15 X icr 3 cm. 



TABLE 10. Dispersion of air. Differential tubes, each 138 cm. long. D line in electric arc. 



=74 cm. 



The values of B found in the first two series of this table, if the standard 

 value of /x i =0.0002927 is assumed, is somewhat small, but as near to the 

 true values as may be expected. Again, if 5Xio 14 =i.6s is assumed, the 

 At i values given in the table are similarly small, being 0.3 per cent short of 

 standard values. A single-tube experiment made for comparison (series 3), 

 similarly, came out too large in each case. It follows from this that p and t 

 observations are not sufficiently guaranteed. It is hardly probable, however, 

 that with a micrometer reading to io~ 3 cm. and estimated to lo" 4 cm. (vernier) 

 the precision can be much enhanced ; for since 



D _\ 2 AAf 76 T 



-(jiei) 



dB 



273 P 



76 r 



dB 



dp 2 e 273 p z 

 or at the C line and D lines, respectively, 



8 P = i cm. 5 = <- 8 * IO T4 



\o-7Xio- 1 



2 e 273 p 



6r=i c 



0.2 Xio- 14 



0.2 XlO- 14 



Now, unless the measurement can be made in terms of rings, it is difficult 

 to detect a few millimeters of pressure difference by displacement only. 



The interesting question now occurs whether the two tubes, if identical 

 for a plenum of air, remain identical (no shift of the interference pattern) 

 throughout all the stages of identical exhaustion. On trial, nothing could be 

 detected, the fringes remaining stationary during the whole period of exhaus- 

 tion, or during the influx of air following a high vacuum. Hence there is no 

 perceptible difference effect of flexure of the glass ends, and the ultimate 

 question of accuracy depends on the measurement of r and p. To eliminate 

 the possible effect of flexure, an air column of negligible length, in which the 

 glass effect only is present, will have to be tested; otherwise there is no possi- 

 bility of separating the air and glass effect. 



