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PROCEEDINGS OF THE AMERICAN ACADEMY. 



and we shall have coincidence at D, to the right of the centre, exactly as 

 before. To test this point experimentally, the interferometer was illu- 

 minated with light from the monochromatic illuminator, a narrow band on 

 the green side of the D lines being utilized. The formation of sodium 

 vapor in one of the paths gave rise to the same changes as were produced 

 in the case of the helium light, it being necessary to increase the air-path 

 to prevent the fringes from disappearing. If the fringes were made very 

 narrow, so as to occupy only a small portion of the field, the wandering 

 of the system to one side could be easily watched, as the sodium vapor 

 was formed. It must be understood that only a very small actual c/is- 

 placeme7it occurs, the wandering of the system being merely a change in 

 position of the region over which fringes can be seen. On repeating tlie 

 experiment with a band of approximately monochromatic light on tlie red 

 side of the D lines, a simihir drift of the position of maximum visibility 



FiGUltE 2. 



was observed, and the direction of the drift was the same as before. In 

 the case of helium light I have been able to increase the path difference 

 to five or six centimeters, or to nearly treble it. 



The achromatizing action of tlie sodium vapor is most beautifully shown 

 if we illumine the interferometer with wliite liglit. 



Under ordinary conditions oidy two or three white fringes are seen, 

 bordered on each side by perhaps a dozen rainbow-colored bands, which 

 fade rapidly into a uniform illutnination. If sodium vapor is formed in 

 one of the interferometer paths, the colored fringes rapidly achromatize, 

 and increase in number, breaking up, however, into groups, as shown in 

 Figure 2. As the density of the vapor increases the number of groups in- 

 creases, each group, however, containing fewer fringes. The position of 

 the centre of the groui)ed system drifts in the same direction, as the point 

 of maximum visibility in the previous experiments. 



The explanation of the altered appearance of the fringes in this case is 

 not as simple as in those previously considered. "We are dealing with two 



