186 



SCIENCE. 



[N. S. Vol. V. No. 109. 



joined in the Funk & Wagnalls' movement to 

 reform their participles in signed articles, and 

 I, therefore, submit a request for permission. 

 G. K. Gilbert. 

 Washington, D. C, 



January 18, 1897. 



AN EXPLANATION OF THE SO-CALLED PSEUDO- 

 AURORA. 



Occasionally, during the winter season, 

 dwellers of our Northern cities have noticed by 

 night a strange optical phenomenon, which some 

 one has called the 'pseudo-aurora,' and which, so 

 far as I know, has not been heretofore ex- 

 plained.* My attention was first called to it 

 some years ago, in Moorhead, Minn. Over 

 each arc lamp, used in street lighting, appears 

 a strange column of pure white light, seeming 

 to extend vertically to a great height ; a peculiar 

 transparent shaft, like the brightest bars of 

 the aurora borealis, yet standing very still, and 

 always vertical over the lamp from whatever 

 point viewed. When each arc lamp in the whole 

 town is thus attended by its vivid shaft the 

 display is magnificent and, seen against the 

 northern sky, might easily suggest the ' pseudo ' 

 name. On an evening of special beauty these 

 columns seem to reach almost to the zeinth, and 

 other sources of light add their shafts to the 

 display. The evening star gives a shaft below 

 as well as above, and the late rising moon stands 

 right in a broad column of light. 



Looking about for causes, and noticing from 

 time to time the conditions under which this 

 meteor appeared, the following facts were ob- 

 served : The temperature is always below the 

 freezing point, oftenest about zero. The sky is 

 cloudless, air still or barely moving, and more 

 or less full of frost crystals. The display is 

 finer, completer, when most crystals are present, 

 though by no means does the mere presence of 

 crystals in the air furnish the spectacle. The 

 shafts of light are most sharply defined and ap- 

 parently higher when the air is stillest. With 

 more wind the shafts spread out, diffuse, be- 

 coming indistinct, and with a gentle breeze the 

 light seems to be more or less evenly distributed 

 through the entire upper air, like a fine luminous 

 dust suspended there. 



* See Loomis's Meteorology, p. 224. 



Having noticed these conditions, it is apparent 

 that the crystals are the important factor, and 

 reflection of light from their facets is suggested 

 at once. Of course to get a vertical shaft of 

 light by reflection necessitates a constant hori- 

 zontal position of the crystal faces, and I 

 searched long and arduously for a ballasted 

 crystal, floating like a parachute, but found 

 none. What I did find in each case was a 

 minute hexagonal plate of solid ice, in no case 

 more than one millimeter in diameter, ex- 

 tremely thin, and of glassy smoothness. 



I experimented with this idea : Making some 

 hexagonal plates an inch across, of the lightest 

 glazed bond paper, and letting them fall in still 

 air from a height, the whole story is told. Each 

 plate floats gently down, at times making a 

 rapid chute edgewise, but quickly recovering a 

 horizontal position, so that of all the time in- 

 volved in falling, the larger part is taken up 

 while the plate is in a position approximately 

 horizontal. We have seen the same thing in 

 autumn when the great basswood leaves let go 

 and float slowly down. 



Now, filling the air with such plates, each of 

 which is a perfect mirror, we have in the ver- 

 tical plane, between our eye and the light, in- 

 numerable crystals, from the lower surface of 

 which rays of light from the lamp are reflected 

 to our eye, and seen by the eye as though lo- 

 cated in the straight line in which they enter 

 the eye, and at a distance equal to the distance 

 traveled from the lamp. This gives the vertical 

 column, the location of any single point in it 

 being shown by construction, the same as an 

 image in a plane mirror. 



The little crystal plate adjusts itself, like a 

 flat stone at the bottom of the torrent, or a 

 cake of ice at the top of the sea, with its broad 

 surface normal to the force acting upon it. So 

 long as this force is gravity only, the position of 

 the crystal is horizontal. But if the wind be 

 blowing this adds a horizontal component, giv- 

 ing with gravity a resultant no longer vertical, 

 to which the plate becomes normal. With the 

 departure of the crystal from the horizontal 

 position the vertical shafts of light disperse. 



J. Paul Goode. 



University of Chicago. 



