102 



G. Bar us — Interferometry of Air 



ences is observed with the telescope at R and the interfering 

 diffraction spectra by the same telescope on being rotated into 

 the position D. The sodium line from the two coincident 

 spectra is an admirable fiducial mark to which the centers of 

 ellipses, as they move across the field, are always referred. A 

 fine horizontal wire across the slit enables the observer to place 

 the spectra in coincidence both horizontally and vertically. 



c^-B 



Fig. 1. 



Fig. 2. 



As in the earlier instrument,* the arms m and n were made 

 of gas pipe, through which a current of water continually circu- 

 lates, when necessary. Tremors of course could not be wholly 

 eliminated ; but the instrument, in spite of its lightness, was 

 made firm by the aid of horizontal and vertical leveling screws, 

 which imparted slight strains to the gas pipe arms m and n ; 

 i. e., the ends at M and JV were pushed away from the wall 

 by horizontal screws until sufficient strain of parts resulted, 

 while at the same time they were lifted above the table carry- 

 *This Journal, xxxiii, 1912, pp. 107-108. 



