MOUNT WILSON OBSERVATORY. 241 



Hitherto, however, only visual observations or low-dispersion prismatic spec- 

 trograms have been made of it. In view of the uncertainty as to the origin 

 and nature of this line and its apparent connection with solar activity, it 

 appeared to Mr. Babcock to be a promising object for study with an inter- 

 ferometer, provided experiment should show that it was sufficiently intense 

 at the latitude of this observatory. 



A Fabry-Perot etalon having plates 1 mm. apart was mounted in front of 

 a hand magnifier of 50 mm. focal length and 25 mm. aperture. Thin films 

 of gold cathodically deposited upon the interferometer plates gave sufficient 

 reflecting power for green light and at the same time transmitted a large 

 portion of it. No dispersing system was used, since no monochromatic 

 radiation other than the green line has been found in the absence of a visible 

 aurora. The first plate was exposed in Pasadena on the night of February 

 25, 1922, for 10 hours and showed a clearly defined ring system. Since that time 

 many photographs have been made both in Pasadena and on Mount Wilson 

 which show the ring system nearly always strong enough for measurement. 

 The constants of the instrument, known with precision, definitely identify 

 the ring system as due to a line in the yellow-green portion of the spectrum 

 and leave no question as to its auroral origin. It is found possible to photo- 

 graph it on nearly every clear night, and even within a few days of the time 

 of full moon, provided a color-screen is employed. Interferometers giving 

 orders of interference of 3,600, 8,600, 18,000, 38,000, and 85,000 for the 

 auroral line have been used up to the present, for the most part with lenses 

 of 75 mm. focal length. A Dallmeyer kinematograph lens working at F/1.9 

 is ordinarily employed, although some of the etalons are not large enough to 

 utilize its full aperture. At the highest order of interference the bright rings 

 are still not wider than one-half of the spectral range of the interferometer, 

 and the resulting upper limit to the width of the line is about 0.035 a. 



The preliminary values obtained for the wave-length average 0.15 a smaller 

 than the mean of the best determinations made with prismatic dispersion. 

 The experience so far gained indicates the possibility of measuring this wave- 

 length with an accuracy of 1 part in 1,000,000 or 2,000,000, unless some 

 evidence of variability is found as the work proceeds. 



As might be expected, large variations occur in the intensity of the auroral 

 light from night to night, the range being perhaps three or four fold. Some 

 correspondence has been noted between its brightness and the state of solar 

 activity. If this is borne out, the return of the period of maximum sun-spot 

 activity should make it easy to secure a large amount of valuable data upon 

 the aurora, even in the latitude of Mount Wilson. 



FIFTY-FOOT INTERFEROMETER TELESCOPE. 



The mechanical and optical problems presented by a 50-foot interferometer 

 are not difficult of solution, but in order to keep the expense within moderate 

 limits, a simple design, involving but little large-scale machine work is essen- 

 tial. The instrument devised by Hale, the details of which have been worked 

 out by Pease and Nichols, embodies the optical features used by Michelson 

 for the 20-foot interferometer in a mounting of apparently the simplest pos- 

 sible type. 



The plane mirrors are carried on a light skeleton girder of structural steel, 

 to be riveted together on Mount Wilson. This girder is 54 feet long and 10 



