CHAPTER X. 



THE DISPERSION OF AIR. 



70. Introduction. In view of the long-armed interferometer available, it 

 seemed interesting to test the refraction of air at different wave-lengths, X. An 

 iron tube of inch gas-pipe, 138 cm. long, was therefore placed in one or the other 

 of the component beams. The tube was closed at both ends by glass plates, 

 about one-eighth of an inch thick, kept in place with resinous cement. A 

 lateral tube communicated with an air-pump and drying train, so that the 

 tube could be alternately exhausted and refilled with air. By using sun- 

 light, the different lines of the spectrum were obtained with sufficient clear- 

 ness, and the method consisted in finding the reading of the micrometer for 

 successive Fraunhofer lines, both for the case of a plenum of air and for a 

 vacuum. If AN is the (monochromatic) displacement of micrometer corre- 

 sponding to the latter difference of pressure, /j. being the index of refraction 

 of air, e the thickness, 



To determine ju x , we must know a/* x / d\. It has been omitted above, because 

 it enters differentially and because of its small value. It appears as a con- 

 stant decrement of AAT X , as X is constant and d/z x / dX is negative. In the present 

 case, where /j. is actually to be measured, cfyi x / dX enters directly and is essen- 

 tial; but it follows from any two experiments when p is found for different 



colors. 



TABLE 8. Values of B. Inch iron gas-pipe, 138.0 cm. long. D line. 



71. Observations with arc lamp. In table 8 results are given as obtained 

 with the electric arc, in which the sodium line usually appears with sufficient 

 distinctness in the spectrum to be available as a line of reference for measure- 

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