THE GRID STRUCTURE IN ECHELON SPECTRUM LINES. 



9 



image of a line. Replacing the echelon shortened the focus for a true 

 narrow echelon image by about 0.6 mm. The focus for the grid com- 

 ponents of the same line was 0.7 mm. shorter yet — the light forming 

 the grid had traversed the echelon plates more than once. 



(5) Although the grid components are generally very well defined 

 (the minimum being " deep"), it is a difficult matter, with a fluctuating 

 source such as an open arc, to obtain accurate measurements. The 

 grid spacings appear to vary slightly at different stages. When the 

 grid is complete the spacing is regular and, within the limits of error of 

 measurement is equal to one fifth the distance between the orders. 

 This was proven as follows :■ — A quartz tube having merely coils of 

 fine brass wire as terminals gave extremely fine zinc lines. Echelon 

 No. 2 was set in double order condition and Ao, the difference between 

 the two orders, measured for X4810 (see Table I). Then the grid was 

 measured as given by a 3 ampere open carbon arc. Three distinct 

 series of readings were taken. Then the tube was again used. The 

 accuracy of an individual setting was about 0.2% in Ao and about 5% 

 in Ag. It thus appears that in this region, at least within 2%, Ao = 

 5Ag. The focus of the instrument was, of course, not changed, the 

 difference between that for primary and secondary maxima being so 

 slight that distances between the components of the grid are not 

 appreciably affected. 



TABLE I. 



Distances are measured in divisions of the'micrometer head. Each Ao dis- 

 tance given is the mean as calculated from four settings; each Ag from two. 

 Settings were made on the six centrally situated grid components. 



A similar series for Zn XG3G2 gave Ao = 27.65 and Ag = 5.6, 5.9, 

 5.5, 5.6, 5.4: mean = 5.5. Hence Ao = 5.0 Ag. 



