444 Prof. P. W. Wood on the 



plates, it is difficult to say positively just what is the actual 

 structure of the lines. This is due in part to the circumstance 

 that, with the most powerful echelons, the distance between 

 the successive orders is less than the width of the group of 

 satellite lines under observation, and in part to the ghosts and 

 false lines which many instruments show. An excellent 

 summary, with charts of the observations made by different 

 observers, will be found in a paper by Gale and Lemon in 

 the ' Astrophysical Journal ' for Jan. 1910. The authors of 

 this paper made a series of photographs of the mercury lines 

 with a large Michelson grating, and though these photo- 

 graphs do not show as much fine structure as has been found 

 by observers working with echelons, they are more con- 

 vincing than any photographs which have been published, so 

 far as I know. 



Having recently installed at my East Hampton laboratory 

 a plane grating outfit of 42 feet focal length, for the study 

 of the iodine absorption spectrum, and as it appeared that this 

 instrument was far more powerful than the one employed by 

 Gale and Lemon, it seemed worth while to make a study 

 of the structure of the lines as shown under the enormous 

 resolving power of this instrument. 



It was found feasible to photograph the blue mercury line 

 (4359) in the fifth order spectrum, for which the resolving 

 power is 375,000 or about that of a large echelon. The 

 green line and the two yellow lines were photographed in 

 the fourth order, for which the resolving power was 300,000. 

 That the grating actually yielded its full theoretical resolving 

 power was amply proved in the study of the absorption 

 spectrum of iodine vapour. 



A description of the grating and the method of mounting- 

 will be found in a previous paper (Phil. Mag. 1912, vol. xxiv. 

 p. 673), and I need only state in the present paper that it was a 

 very excellent plane grating ruled by Dr. Anderson on the 

 15,000 machine, ruled surface 3 J x 5 inches (or 75,000 lines in 

 all) mounted in the Littrow form behind a six-inch achromatic 

 lens, kindly placed at my disposal by Professor Campbell of 

 the Lick Observatory. The focal length of the instrument 

 was double that of the one used by Gale and Lemon, and the 

 resolving power of the grating was certainly three times as 

 great as theirs, as will be seen from a comparison of the 

 photographs which accompany this paper with theirs (fig. 4 

 of PI. IV. in particular). 



The source of light was a Cooper-Hewitt mercury arc, used 

 "end-on.'"' The use of the tube in this position materially 

 shortens the exposure, without giving rise to any reversal 



