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VII. On Rainbows. By John Tyndall, F.R.S* 



THOUGH the following brief communication contains 

 nothing new in principle, its novelty from an experi- 

 mental point of view, and its possible utility to the teacher as 

 a means of illustration, induce me to publish it in the ' Philo- 

 sophical Magazine.' 



At 9 p.m. on Thursday, September 27th, I opened one of 

 the doors of my cottage in the Alps. It was a very dark 

 night, and the air outside was filled with fog and drizzle. In 

 the corridor behind me hung a small lamp, and as I stood in 

 the doorway the shadow of my body was cast upon the fog. 

 Such a shadow I had often previously seen, but on this occa- 

 sion it was accompanied by an appearance that I had not seen. 

 At a considerable distance from the shadow, and far beyond the 

 illuminated portion of the fog, a well-defined luminous circle, 

 of which my head was the centre, was projected upon the dark 

 background. Various changes in the position of the light 

 and of my body were made, with a view of enhancing and 

 examining the effect. On walking forward from the doorway 

 into the fog both shadow and halo went in advance of me. A 

 more powerful lamp produced a brighter circle ; but a single 

 candle sufficed to render it visible. On the 29th, the atmo- 

 sphere being in the same condition, the circle was well ob- 

 served from another door. On the 18th of November, and 

 subsequently, I observed the same phenomenon from the door 

 of my hut upon Hind Head. 



On first observing the shadow and circle combined, I 

 thought the effect a reproduction, on a small scale, of the 

 spirit of the Brocken, and wrote to Professor Stokes to this 

 effect. Having no means of measuring accurately the angle 

 subtended by the diameter or radius of the luminous circle, I 

 placed two crossed laths in such a position that the prolonga- 

 tions of both intersected the circle at the opposite ends of the 

 same diameter. The angle measured in this rough way was 

 clearly under 90°, though not much under it. Between the 

 luminous circle and the shadow I could see no trace of colour. 



These and other facts caused me to suspect that the circular 

 band was really a rainbow ; and on my return from the Alps 

 I tried to reproduce the effect. A cylindrical copper boiler, 

 16 inches high and 12 inches in diameter, was charged with 

 water, which had its temperature raised by gas-burners till a 

 pressure of 20 lbs. was obtained. On opening a valve the 

 steam, carrying droplets of water mechanically along with it, 



* Communicated by the Author. 



