Outside the rainbow there was part of a secondary bow. The Ochil Hills were north of his point of observation. These hills were covered with snow, and the setting sun was glowing with rosy light. Never had he seen such a depth of colour as was on them on this occasion. It was a deep, furnacy red. The sun's light was shorn of all the rays of short-wave length on its passage through the atmosphere, and only the red rays reached the earth. The reason why the Ochils glowed with so deep a red was owing to their being overhung by a dense curtain of clouds, which screened off the light of the sky. The illumination was thus principally that of the direct softer light of the sun. CHAPTER XXI THE AURORA BOREALIS He must be a very careless observer who has not been struck with the appearance of the streamers which occasionally light up the northern heavens, and which farmers consider to be indicators of strong wind or broken weather. The time was when the phenomenon was considered to be supernatural and portentous, as the chroniclers of spectral battles, when "fierce, fiery warriors fought upon the clouds, in ranks and squadrons, and right form of war." And even in the rural districts of Britain, the blood-coloured aurora, of October 24th, 1870, was considered to be the reflection of an enormous Prussian bonfire, fed by the beleaguered French capital. In joyful spirit, the Shetlanders call the beautiful natural phenomenon, "Merry Dancers." Burns associated their evanescence with the transitoriness of sensuous gratification:--"they flit ere you can point their place." And Tennyson spoke of his cousin's face lit up with the colour and light of love, "as I have seen the rosy red flushing in the northern night." Yet this phenomenon is to a great extent under the control of cosmical laws. One of the most difficult problems of our day has been to disentangle the irregular webwork of aurorae, and bring them under a law of periodicity, which depends upon the fluctuations of the sun's photosphere and the variations on the earth's magnetism, and which have such an important influence upon the fluctuations of the weather. The name "Aurora Borealis" was given to it by Gassendi in 1621. Afterwards, the old almanacs described it as the "Great Amazing Light in the North." In the Lowlands of Scotland, the name it long went by, of "Lord Derwentwater's Lights," was given because it suddenly appeared on the night before the execution of the rebel lord. In Ceylon aurorae were called "Buddha Lights."