134 



THE INTERFEROMETRY OF 



CC. As the walls of the brass pipe were thick and the ends relatively short, 

 there seemed to be no objection to this arrangement. Care was taken to 

 conduct the escape steam and hot gases away from the interferometer. 



The displacement interferometer was of the linear type described above, 

 the mirrors M and N and the grating G being attached directly to the wall of 

 the pier and without an intervening rail. Unfortunately the pier in a large 

 city is also in incessant vibration, so that the interference patterns quiver. 

 It is this insuperable difficulty which has prevented me from reaching results 

 as accurate as were anticipated. A few of the data, however, will be added 

 as an example of the efficiency of the method. 



TABLE 14. Refraction of air at different temperatures. Tube, 71.7 cm. long, 



2.5 cm. in diameter. 



79. Observations. The data are given in table 14, where the temperature 

 and barometric pressure are shown in the first column, the differences in the 

 pressure p between the plenum of air and the exhausted air in the refraction 

 tube in the second column, while the third shows the values of A-/V, the dis- 

 placement of the micrometer corresponding to p, as found in successive inde- 

 pendent experiments at the temperature given. For such long distances 

 between grating and mirrors the ellipses are visually distorted, and much 

 depends on finding a satisfactory sharp interference pattern. This was the 

 case, except in series 3 and 5, when for incidental reasons (outside tremors) 

 the patterns were disagreeably flickering. The observations are usually for 

 room air, as the special drying of air in series 3 and 5 made no perceptible 

 difference. At 100 care must be taken to obviate convection currents of 

 air, so far as possible. The endeavor was made to keep p as nearly as possi- 

 ble at the same value, apart from the barometer pressure, which does not 

 enter into the equations. In series 4 the values of AN" are relatively large, 



