THE GRID STRUCTURE IN ECHELON SPECTRUM LINES. 



15 



components increases, the first step resembling a narrow reversal as 

 in Figures 8 and 10 or a central fixed line with two moving wings as in 

 Figures 7 and 9. But the writers feel that this apparent motion is 

 due to the fact that each grid component is not formed in ioio at once: 

 the part which lies nearest the center of the system is formed first. 

 Certain it is that this apparent motion ceases abruptly when the 

 component has reached a position which is one grid distance from 

 its neighbor. If the source be an arc, many rapid fluctuations in 

 intensity occur. 



(h) Although the resolving power of the grating (225,000 in the third 

 order) is far below that of the echelon (about 750,000 for X6100 for 

 echelon No. 2) it is hard to reconcile the images given by the two 

 instruments on any other assumption than that the grid is due to 

 secondary action. 



To throw further light on the problem, Li X6104, given by a vertical 

 carbon arc soaked with LiCl, was viewed simultaneously by echelon 

 and grating. Table II gives a summary obtained from various ar- 

 rangements. 



TABLE II. 



1 1 1 1 indicates the grid; ■ a broad structureless line; | a narrow unreversed 

 line, or one very slightly reversed; || a broad and strongly reversed line. 



Therefore which pole is soaked makes no difference, nor does it 

 matter which pole is above. The region near the + pole generally 

 shows the grid in the echelon, that near the — pole a broad structure- 

 less line. The grating always gives a narrow unreversed line or one 

 very slightly reversed where the echelon shows the grid, and a strongly 

 reversed line where the echelon shows no structure. Thus the grid 

 does not result from conditions which produce a reversed grating line. 



