auroral display; but some of the brightest have been seen at full moon. Some have said that auroras could be seen in daylight, but this sounds like a ghost story.
It has been almost impossible to photograph the aurora, although in the winter of 1882–'83 a Swede at Spitzbergen claimed to have secured a faint image in eight minutes and a half. This was done before access was had to the orthochromatic plates, color screens, or lightning emulsions of to-day.
Some observers claim that the stars scintillate less when seen through an aurora; but this is caused, according to Montigny, by the presence of fog. On the contrary, it is clearly proved that the scintillation increases during any magnetic disturbance, even when the latter is not accompanied by an aurora.
As to the study of the nature of the light of an aurora there are two methods: by the polariscope and by the spectroscope. By the former it is easy to recognize whether it is a natural light emanating from a self-luminous body, or whether it reaches the eye after undergoing one or more refractions or reflections. Biot, in 1817, in the Shetlands, could not discover the smallest trace of polarization; this has been confirmed by Macquorn, Rankine, and Nordenskiold, and proves that the light of the aurora is not, like that of rainbows and halos, the result of reflection or refraction, but is itself luminous.
This important discovery is confirmed by the spectroscope. If the light emanating from a solid or liquid incandescent body is passed through the spectroscope, the resulting spectrum is continuous; if, on the contrary, the source of the light is gaseous, the spectrum is composed of a certain number of bright lines or stripes separated from each other by dark intervals. The number, the position, and the brilliancy of these bright lines depend upon the chemical constitution of the glowing gaseous body. The spectrum of the aurora was studied by Angstroem, in 1866, for the first time, and is essentially a spectrum of lines, hence gaseous by nature; it can not, therefore, be due to a reflection of the light of the sun, as has been supposed. The spectrum of the aurora runs the gamut from red to yellow, green, blue, and even violet, the last line of the spectrum having been seen but once, by Lemstroem in Lapland. Some of the lines are very similar in position to the spectrum of the electric spark or of lightning. The fourth line has not been found in any known body, and Angstroem attributes it to phosphorescence or fluorescence. For instance, oxygen is phosphorescent, and there is an abundance of ozone created by the aurora. A drop of sulphate of quinine has been made luminous by the action of the rays of the aurora, and so also has the double cyanide of platinum and potassium.
But, although we are getting nearer to the truth as to the nature
