98 PROCEEDINGS OF THE AMERICAN ACADEMY. 



we may call the Pfund arc/^ between two iron rods, the upper, the 

 negative terminal, being 5 mm. in diameter and pointed somewhat, 

 and the lower, the positive, being 16 mm. in diameter, the current 

 varying from about 1 to 9 amp. and the E. M. F. of the circuit being 

 220 volts; and the other a 110 volt circuit arc between carbon termi- 

 nals, the lower being positive, and the values of the currents used 

 being within the above limits. In both cases the positive terminal 

 was supplied with small pieces of the necessary metal, ordinary com- 

 mercial zinc. The echelon image was magnified about 3f diameters 

 by a Bausch and Lomb microscope. 



Two shutters were used, at first a very light wood and wire arrange- 

 ment, having two sets of openings of three and two openings respec- 

 tively, placed in the focal plane of the echelon spectroscope; and 

 finally a shutter of cardboard, having two sets of openings of two and 

 one openings respectively, placed over the slit of the constant devia- 

 tion spectroscope (this method giving good results, as the echelon 

 spectroscope slit was set accurately in the focal plane of the telescope 

 of the constant deviation spectroscope) . The echelon was covered with 

 a cotton lined box to prevent temperature changes, which were never 

 more than 0.1° C during any one set of exposures and usually much less. 

 The photographic plates generally used were Seed Gilt Edge # 27, in 

 some cases double-coated; the developer generally normal rodinal 

 solution. 



In adjusting and testing the echelons a Cooper-Hewitt mercury 

 lamp, kindly loaned by Mr. William Sawtelle of Harvard, was used. 



The two inductance coils used were as follows : — 



(a) A coil having three layers as described on page 186 of the Astro- 

 physical Journal for October, 1905. 



(b) A commercial coil of annunciator wire, weight about 8 lbs., 

 size of wire # 18 S. W. G. 



The arrangement of the apparatus is shown in the figure. 



<2A 



^ .^ oir^io ^ 



S, S, slits; P, prism; 0,0,0,0, lenses; G, echelon grating; E, eye. 



17 Pfund, Astrophysical Journal, 27, 296, May (1908). 



