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XXXVI. On certain Phenomena accompanying Rainbows, By 

 Silvanus P. Thompson, D.Sc.,B.A., F.R.A.S., Professor of 

 Experimental Physics in University College, Bristol*. 



The literature of the Rainbow is somewhat extensive ; yet 

 I do not recollect having anywhere seen any record of 

 certain phenomena, of radial streaks of light, which are occa- 

 sionally to be seen accompanying rainbows. I am convinced 

 that the phenomenon is not rare ; and in the desire of inducing 

 more observers to watch for its occurrence, I beg to present 

 the few notes which follow. On the evening of July 8th, 

 1877, I stood with a friend upon the summit of the Drachen- 

 fels about an hour before sunset. To the south and south- 

 east the sky was obscured by dark masses of cloud ; and the 

 sky above us and to the north and west was covered with 

 broken clouds : there was no perceptible wind. A fine 

 rainbow spanned the valley, and extended eastwards toward the 

 Lowenberg. Both primary and secondary bows were fine ; 

 and several supernumeraries were noticed. As the bow faded 

 away a faint streak of light was observed, not unlike the 

 streak of an aurora, outside the right limb of the secondary 

 bow, extending up to it, about 12° of arc long by 2° wide, 

 and making an angle of about 15° with the horizon. It lasted 

 fully ten minutes, maintaining nearly same position and form. 

 It was perfectly free from colour. A similar, fainter streak 

 was observed on the left limb at about 70° to the horizon. 

 Standing upon the very summit of the Drachenfels, the pro- 

 file of the hill and our shadows were rudely projected upon 

 the nearest hillside ; it was very easy, therefore, to verify that 

 these streaks were really radial in direction. Each of them, 

 if produced backwards, would have passed through the point 

 of space exactly opposite the sun. 



July 15th, 1877, after a very wet and stormy day, we ob- 

 served, from the promenade in front of the Schweitzerhof at 

 Lucerne, a very fine rainbow at sunset. The colours were 

 unusually brilliant. Seven supernumeraries of the primary 

 bow were distinctly visible, and one of the secondary. The 

 clouds behind presented a uniform deep grey tint. A colour- 

 less streak was visible within the primary bow, radial in posi- 

 tion and at about 45° to the left. 



July 16th, 1877, about 6.30 p.m., from the high road near 

 Sarnen, the same observers watched the same phenomenon. 

 The day was thundery and with frequent showers and gusts 

 from the north. A very fine and complete bow was seen, 



* Communicated by the Author, having been read before Section A of 

 the British Association, Aug. 19th, 1878. 



