On certain Phenomena accompanying Rainbows. 273 



partly against the sky, partly against the hills which bound 

 on the south-east the high road from Alpnach. At intervals 

 several radial streaks were observed both interior and exterior 

 to the rainbow. They shifted position and magnitude rapidly. 

 It was noticeable that to each streak corresponded a patch of 

 sunshine on the hills behind, each streak pointing to a patch 

 and moving with it. 



Three times in June 1878 1 observed at Bristol similar phe- 

 nomena accompanying rainbows, though not so favourably. 



The explanation which is suggested by the observation made 

 at Sarnen is very simple. These wedge-shaped radial streaks 

 are "beams" of sunlight, and become visible by diffuse re- 

 flexion from particles of matter in their path, just as the appa- 

 rently divergent beams of sunrise or sunset become visible. 

 These " beams " being practically parallel to one another, 

 appear to converge in the point exactly opposite the sun by 

 perspective ; or, in fact, just as the parallel beams of sunsot 

 appear divergent. Since the rainbow has for its centre the 

 point opposite the sun, such beams must necessarily have 

 positions radial with respect to the bow. They resemble, 

 therefore, the "rayons du crepuscule" occasionally seen in the 

 east at sunset, or in the west at sunrise. 



Here let me mention one peculiarity of the radial streaks, 

 in which they appear to differ from the "rayons du cre- 

 puscule." I have never observed a " streak " crossing the dusky 

 region between the primary and secondary bows, though I have 

 seen one and the same streak extend beyond the bows both 

 outside the secondary and inside the primary. I have never 

 seen a " streak " of colourless light cross the coloured part of 

 a rainbow. I have noticed a faint bow crossed by a streak ; 

 where the streak crossed it, the bows (primary, secondary, and 

 supernumerary) became more vivid. There was more light ; 

 but it was dispersed in the usual way, and the intermediate 

 region between the bows was no brighter where the streak 

 crossed it. I conclude, then, that such a streak is the very 

 stuff, so to speak, of which rainbows are made ; only when 

 the neighbouring regions are obscured by clouds, the beam of 

 light which struggles through builds up its own portion of the 

 arch in its appropriate place. 



Two not dissimilar phenomena (quite independent of the 

 rainbow) appear to confirm this conclusion. Stand in the 

 sunlight, when the sun is high, so that the shadow of your head 

 falls upon the surface of a slightly turbid pond or lake, whose 

 surface is covered with gentle waves, or ripples : you will 

 see the shadow of your head surrounded by a halo of quivering 

 radial streaks of light. From the deck of a steamer on the 



Phil Mag. S. 5. Vol. 6. No. 37. Oct. 1878. T 



