14 KENT AND TAYLOR. 



ously by echelon No. 2. For Zn X4810 three components of the grid 

 exist when the grating shows a line 0.12 t. m. broad. Ao for X4810 = 

 0.155 t. m. f X 0.155 = 0.09 t. m. which compares favorably with 0.12 

 t. m. The complete grid exists when the line is 0.3 t. m. or 2Ao 

 broad and the image begins to pass into a structureless line at 3Ao. 

 Similarly for Li X6104 a full and well-marked grid exists at a line 

 width about 0.2 to 0.5 t. m. or Ao to 2Ao (as here Ao = 0.25 t. m.). 

 The grid is poorly marked above about 2Ao and is gone at 3Ao. 



(c) Numerous lines in the spectra of Na, Hg, Fe, Mg, Cd, Ca, Sn, 

 Pb, and Bi, developed by an open carbon arc, show the grid whenever 

 the line is sufficiently broad — rendered so by introducing more of 

 the substance or increasing the current; also by increasing the capac- 

 ity in the case of a spark. 



\d) LiXXG708 and 6104, Zn XX4810, 4722 and 4680, also HgX5461 

 (mercury being fed into the lower cored carbon) show by their behavior 

 that a line which is too broad will appear structureless in the echelon, 

 that the center of the core of an arc may show the grid complete while 

 light from the wings of the image gives a simple structure of but one 

 to three components. With a sufficient amount of vapor the com- 

 plete grid may be obtained even at low pressure. 



(e) A study of Zn X4810, from an arc in the vacuum or pressure tank, 

 at pressures from 2 cm. of mercury to about three atmospheres, 

 showed that moderate changes of pressure do not produce measurable 

 displacements in the grid components, but merely alter somewhat 

 their relative intensities, shifting the maximum over one or two com- 

 ponents or even bringing up new ones. This of course means that, 

 as long as a grid exists, the components do not change appreciably 

 their position with changes of wavelength as small as 0.015 or 0.020 

 t. m.^ Their position is affected more strongly by the position of the 

 echelon and its temperature. Similarly, the grid components of the 

 spectroscopic doublets Li XX6708 and 6104 developed in vacuum tubes 

 show intensity shifts with changes of pressure over the range of one 

 atmosphere. 



(f) The " end on " position of a vacuum tube will generally show a 

 more complete grid than that "side on." 



(g) If a line broaden unsymmetrically with increase of current the 

 maximum of intensity will shift. Those components which are just 

 being formed show an apparent motion outward as the number of 



5 According to Humphrey's and Mohler's results for Zn, the pressure shift 

 reduced to X 4000 is 0.057 t. m. for twelve atmospheres. 



