THE GRID STRUCTURE IN ECHELON SPECTRUM LINES. 



13 



occurring in the position of the narrow tube line, as in Figure 8, for a 

 double order condition of the echelon, there are nine or even eleven 

 components when the grid is strong. Note that the grid components 

 2, 3, 7, 8, which at first are very brilliant when the grid is "young," 

 grow weaker, 2 and 8 often being so faint that it is difficult to make 

 accurate micrometer settings upon them. 



Case III: — The treatment is the same for a single order condition 

 of the echelon, as in Figure 9 which shows a triplet, quintuplet, or, 

 with neighboring parts of adjacent orders, even as many as eleven 

 components. 



Case IV: — Here a grid minimum coincides 

 with the primary maximum and the grid com- 

 ponents are as shown in Figure 10. 



The above statements explain why an origi- 

 nally narrow line, as its width increases, may ap- 

 pear, as it actually does, a triplet or quintuplet, as 

 in Figures 7 and 9, or may, as it were, "reverse" 

 and then quadruple, as in Figures 8 and 10. Actual 

 reversal as shown by the grating probably occurs 

 much later in the historv of the line. (See page 



15.) 



Further, if a line be intrinsically unsymmetri- 

 cal, shading off to the red for instance, the sec- 

 ondary action masks an early stage of broadening, 

 and the left grid line, 2, forms as in A, Figure 11. 

 Line 3, as in A', then comes up as 2 strengthens. 



(b) The grid begins to disappear and the line 

 gradually becomes broad and structureless when 

 the primary line exceeds 2Ao in width, Ao being 

 the distance between two adjacent orders. This 



was determined as follows : — Using 



as narrow a slit as possible, a low 



power ocular and a mm. scale, an 



eye estimate was made of the 



breadths of various portions of an 



arc line shown by the grating. 



These were reduced to t. m. The 



same source was viewed simultane- 



Fic. 10. 



Figure 10. Case IV: Echelon in single order condition and 

 a grid minimum coincident witii the primary maximum. 



