G Barus — Interferences of Crossed Spectra. 495 



gular axes. To put the mirrors M and N in parallel and in the 

 vertical plane with the grating G, the half silver plate should 

 be removed and replaced by a small white vertical screen of 

 cardboard, placed at right angles to the direction of US in 

 figure 4, and receiving both spectra. A fine wire is drawn 

 across the slit to locate the longitudinal axis and an extra lens 

 may be added to the collimator and properly spaced until the 

 doublet insures sharp focusing. Both mirrors M and JY are 



10 



<S 2)' 



now rotated on horizontal axes, until the longitudinal black 

 lines in their spectra cease to diverge and coincide accurately. 

 G,M,N may now be considered in adjustment. On returning 

 the half silvered plate, IIS, it in turn is to be carefully rotated 

 around horizontal and vertical axes, until the horizontal black 

 line in the spectrum and the sodium line (always incidentally 

 present in the arc lamp) both coincide. But as a rule it will 

 be found that if the longitudinal axes ww, figure 5a, coincide, 

 the D lines cross each other at a small angle, exaggerated in 

 the figure. The interferences, when found by moving the 

 micrometer at M, are usually coarse irregular lines, indicating 

 a center not very distant and located on the level of a band 

 where the D lines cross. 



On the other hand, if the D lines are brought to coincidence 

 by moving the adjustment screws on M and N (which throws 

 them out of parallel), the longitudinal axes tow, w'w', figure 

 oh, diverge at a small angle and the interferences are found in 

 a vertical band where the lines ww and w'w' cross. This 

 band is relatively wide however as compared with the cases in 



