152 



THE INTERFEROMETRY OF 



correct. This can not be referred to the temperature of the 4-inch tube, 

 because the initial optic density is necessarily measured. It must therefore 

 be due to the temperature of the vacuum chamber. It follows, therefore, 

 that the time allowed in these experiments, between observations, though 

 sufficient for establishing the initial temperature of the air-chamber, is not 

 sufficient for the much larger vacuum chamber. The two chambers are thus 

 no longer at the same temperature, a condition which the equations implicitly 

 assume. 



The apparatus was now taken apart and thoroughly overhauled. After 

 reassembling the parts, the chamber was found free from leakage. As the 

 exhaust pipe was in the way of the beam of light entering the telescope, the 

 offset, consisting of two parallel mirrors firmly adjusted, was used without 

 annoyance, here as above. The work throughout progressed smoothly, though 

 the ellipses were again not as satisfactory in form as would have been desirable. 



TABLE 21. Values of 7. Data as in Table 11,4" brass pipe. 



Table 21 contains the results. Changes in the values of AAT in a given 

 series are most likely referable to the form of the interference pattern, indi- 

 rectly to the flickering of the electric lamp. There seems to be no evidence 

 to associate them with the manner in which the 2^-inch stopcock is opened 

 and closed. This was merely jerked around 180, between the two closed 

 positions of the plug, and, so far as can be seen, the rate of motion is adequate. 

 The successive observations show no consistent difference, as was the case 



