THE GRID STRUCTURE IN ECHELON SPECTRUM LINES. 11 



and double order there are four lines. It is probable that these four 

 lines, under conditions of inferior illumination, were interpreted as 

 four separate and true lines. It is unfortunate that at first the only 

 line available for study was a doublet. With this latter and better 

 source a zinc chloride solution gave X4810 sufficiently strong. The 

 crossed dispersions prove it to be a simple, though broad, single line 

 when the echelon alone shows the grid. 



Further Results: Characteristics of the Grid. 



(a) From numerous observations upon Li X6104 and Zn X4810, as 

 developed by various sources, such as vacuum tubes and arcs (on 110 

 and 220 volt D.C. circuits and from 1 to 20 amperes) under high, 

 normal and low pressure, in which the cross-hairs of the filar micro- 

 meter were set successively upon the true, narrow, lines given by the 

 tube and the grid components given by the arc, it is quite certain that 

 the grid is built up approximately as follows: — Suppose that in a 

 hypothetical grating of resolving power and dispersion equal to that 

 of the echelon, a line which is at first very narrow, e.g., 0.025 t. m., 

 gradually becomes less monochromatic, owing to changing conditions 

 in the source, and appears as represented diagrammatically by the 

 small letters a to e, Figure 7. Four cases must be discussed as shown 

 in Figures 7 to 10, respectively. 



Case I: — The echelon in double order condition gives successively 

 images A to E. When the line is very narrow the echelon shows it as 

 such, in A. Similarly for a line of width, Ag — the width of a grid 

 component or an intergrid distance — it is shown as in B. When of 

 width 3Ag, the echelon shows no change, C appearing as B; for, at m, 

 the primary and secondary action together give a decided minimum. 

 When the line has a width, as in d, the echelon shows a triplet, D, and 

 when of width as in c, or greater, the grid is complete — five grid 

 maxima, 1 to 5 and 6 to 10, for each maximum, such as 3 and 8, which 

 a narrow line would give; four maxima, 4 to 7, between the double 

 order positions, 3 and 8, of such a narrow line. For a given position 

 of the echelon these grid components do not, in forming, move very 

 much, if at all: they come up in situ. There exists an apparent 

 motion, in and out, which is probably due to the changing width of 

 the primary line, which may not at all times be such as to complete 

 the entire width of a grid component. 



Case II: — When the position of the echelon, its temperature and 

 the wave length of the line observed, result in the central grid minimum 



