420 C. Barns— Interference of Reversed Spectra. 



trial. In my case the distances to the line of mirrors NM 

 were 165 cm for G and 90 cm for G'. This method automatically 

 excludes the direct beam and all glare and gives excellent 

 spectra, both in the first and second orders. The use of two 

 gratings, however, introduces the difficulties of adjustment 

 specified, as the two D doublets corresponding to j¥ and M 

 will not as a rule be parallel and normal to the longitudinal 

 axes of the spectrum, unless all cardinal features (like the 



^6 

 a a 2),' 



a 



Cs 



d 



f 



rulings and their planes) are quite parallel. If the grating is 

 not normal to the impinging beam, the axes of the correspond- 

 ing spectrum is a curved line. As a result I was not, in my 

 earlier work, able to produce the phenomena with two gratings, 

 after many trials, in spite of the clearness of the overlapping 

 spectra. 



Later, having added a number of improvements to facilitate 

 adjustments, I returned to the search again and eventually 

 succeeded. There are essentially four operations here in 

 question, supposing the grating G approximately in place. 

 By aid of the three adjustment screws on each of the mirrors 

 M and JY, figure 5, the fine wire drawn across the slit may be 

 focussed on the grating, if an extra lens is added to the col- 

 limator and the black horizontal shadows of that wire, across 

 the corresponding spectra, placed in coincidence. The grating 

 G' is then to be moved slowly fore and aft, normal to itself, on 

 the slide, so that the position in which the sodium lines are 



