September 2, 1921] 



SCIENCE 



185 



lieve the first time it has been photographed 

 and the negative should furnish much the more 

 accurate determination of its wave-length. 

 V. M. Slipher 

 LffWELii Observatory, 

 Flagstaff, Arizona, 

 May 20, 1921 



The magnificent auroral display seen at the 

 Lowell Observatory on the evening and night 

 of May 14 was of much interest to observers 

 in a latitude so far south on account of the 

 great brilliancy and diversity of types of many 

 of the formations and their widespread dis- 

 tribution over the sky. The auroral light 

 was first recognized about 8 :25, or a little 

 earlier, before twilight had disappeared. The 

 lower sky was then very brilliant in the north 

 and particularly in the northeast, and the 

 characteristic greenish auroral color was pre- 

 dominant but over parts of the active areas a 

 suffused ruddy glow was conspicuous, espe- 

 cially in the northeast. Almost immediately 

 streamers made their appearance in the north 

 and northeast. Ten or fifteen minutes later 

 the display developed into great activity. Long 

 brilliant streamers were reaching up towards 

 the zenith and beyond, and at the same time 

 occurred in various parts of the sky, but espe- 

 cially in the east and west, brilliant patches, 

 and masses of light suggesting unevenly illu- 

 minated cloud forms as when cumulus masses 

 are lighted on one side with the great bulk 

 of the cloud thrown into relief by the parts 

 that are more feebly illuminated or in the 

 shadow. One was inclined at first to attribute 

 these massive forms, the darker parts barely 

 perceptible against the background of the sky, 

 as being partially due to atmospheric clouds. 

 But their auroral origin soon became evident. 

 The entire formation vanished when the au- 

 roral activity in that region ceased, and also, 

 with some attention, it was possible to dis- 

 tinguish the few inconspicuous ordinary atmo- 

 spheric clouds present. Several of the bril- 

 liant patches persisted in nearly the same po- 

 sition for some time, fading out and bright- 

 ening up again repeatedly. 



The greatest activity of the second outburst 



occurred between the hours of 10 and 11. Au- 

 roral formations were at that time visible in 

 practically all parts of the sky, exhibiting 

 simultaneously streamers, luminous masses, 

 and bright patches, and all undergoing inces- 

 sant change. Streamers from every direc- 

 tion were playing across the heavens, the 

 great beams of light gradually becoming nar- 

 rower on approaching the region of con- 

 vergence. 



The activity in the region near the con- 

 vergent was at times quite remarkable. The 

 transformations were complex and rapid, the 

 luminous detail flaring up and fading out in 

 almost the twinkling of an eye in some in- 

 stances. About 10 :46 occurred a very striking 

 display in this area when detail formed and 

 dissolved at an extremely rapid rate, struc- 

 ture appearing in momentary flashes and at 

 one time suggesting the fragments of a par- 

 tially formed crown. 



The term " convergent " will be used in 

 connection with the phenomena of the stream- 

 ers, as I had frequently the impression that 

 the streamers from different directions did 

 not radiate from the region of their concur- 

 rence but in many cases took a perceptible in- 

 terval in rising from the lower parts of the 

 sky to the point in question, gradually ap- 

 proaching it in a series of intermittent or pul- 

 sating advances. Doubtless the apparent con- 

 figuration is a matter of perspective as in 

 the case of meteor paths. The streamers de- 

 scending along the lines of force of the 

 earth's magnetic field are for any locality 

 nearly parallel and the vanishing point — the 

 point where the streamers appear to meet — 

 would be the highest altitude at which the 

 streamers become visible. Strictly speaking, 

 it might be more definite to use the term ra- 

 diant as understood for meteors. 



The writer made several estimates of the 

 convergent of the streamers but he missed 

 many opportunities for additional estimates as 

 well as for observing and recording numerous 

 other phenomena in attempting to photograph 

 the streamers in the region of their concur- 

 rence. At this point, or small area, the stream- 

 ers were much of the time comparatively faint 



