THE GRID STRUCTURE IN ECHELON SPECTRUM LINES. 7 



diameter, and sealed in with De Khotinsky cement, each joint being 

 cooled by a water jacket. The salt is shoved into the capillary by a 

 wire and the tube will run many hours without refilling. It may be 

 used end on as well as side on. The capillaries varied from 2 to 0.5 

 mm. 



Auxiliary apparatus as shown schematically in Figure 5. The bulb 

 B, prevented too rapid changes in pressure. The system was washed 

 out with hydrogen from a Kipp generator, dried by sulphuric acid and 

 a calcium chloride tower. The mercury manometer, IVI, indicated the 

 pressure — generally from 8 cm. to a fraction of a millimeter. 



Procedure and Certain Results. 



Both Lummer plates were each in succession crossed with echelon 

 No. 1. In each case, with a carbon arc soaked with lithium chloride, 

 both at atmospheric pressure and in a moderate vacuum, there 

 appeared a pattern which, at this stage of the investigation, seemed 

 to indicate that the grid was real. The following facts, (1) to (6), are, 

 however, clearly not in accord with this conclusion, and prove con- 

 clusively that this curious structure is due to the phenomenon of 

 "secondary maxima" observed by Stansfield ^ and resulting from 

 successive reflections from the surfaces of the echelon plates, producing 

 a Fabry and Perot system in the region of the primary light of the 

 echelon. (1) to (3) deal with some of the criteria of echelon secondary 

 maxima given by Stansfield. These criteria are, in essence, indicated 

 below by italics. 



(1) The width of LiX 6104, given by an open carbon arc at atmos- 

 pheric pressure, as seen in the Littrow grating, using a narrow slit, 

 was found to be about 0.25 t. m. when echelon No. 2 showed the grid 

 plainly. The suspicion, therefore, was confirmed that the line was 

 too wide for the echelon, the difference between the adjacent orders 

 being about 0.26 t. m. In the case of Janicki's observation * of 

 Hg. X 5461, Nutting's work on lines of many elements, and the work 

 of one of us on the zinc lines as given by arc and spark, the indications 

 are that with all lines for which the echelon shows the grid, their 

 breadth is so great that the use of this instrument is not at all justifi- 

 able. 



The writers then proceeded to study the structure from this new 



3 Phil. Mag. (6) 18. .383. 1909. 



* An. der Phys. Vol. XIX, p. 36. 1906. 



