148 Prof. Tyndall on the White Rainbow. 



mountains. Probably the conditions for such an observation 

 do not exist on the earth's surface. 



In conclusion, I may be allowed to express my regret at 

 not being able to make a contribution of positive value to the 

 investigation of this subject. The state of the question is about 

 this : — A well-founded theory of terrestrial temperature can 

 be built only upon an accurate knowledge of the laws of 

 emission and absorption of radiant energy of different wave- 

 lengths, especially in the atmosphere, and the result will 

 appear as a numerical calculation, more or less exact, of the 

 temperature resulting from assigned conditions, and not as the 

 conclusion of an argument to show one thing or another. 



XIX. Note on the White Rainbow. 

 By John Tyndall, F.R.S.* 



THE highest portion of Hind Head Moorland, which is 

 close to the spot where I now write, is about 900 feet 

 above the sea. Over this high plateau, at the present season 

 of the year, mist and fog frequently settle. This has given 

 me of late frequent opportunities of observing, on dark nights, 

 the white circular bows described in my last brief communi- 

 cation to the Philosophical Magazine. All fogs are not able 

 to produce these circles. What is usually known as " Scotch 

 mist" appears eminently suitable for this purpose. 



Last Christmas Day was heralded by mist and fog of a very 

 dense character. Both, however, became thinner as the day 

 advanced ; and finally a blue sky overarched the moorland. 

 Accompanied by my wife I walked out upon the common. 

 When facing the sun we noticed the air alive with small 

 glistening specks. They were an obvious residuum of the fog, 

 produced probably by the coalescence of its smaller particles to 

 minute drops. My previous experience led me to infer that these 

 specks or globules must be able to produce some kind of rain- 

 bow. The inference was immediately verified. Turning my 

 back to the sun, and looking across the Devil's Punch Bowl, 

 a well-defined white bow was seen spanning that remarkable 

 basin. The sky above us was blue at the time, the fog had 

 disappeared, and nothing which could be properly called rain 

 existed in the atmosphere. 



Circular bows are sometimes seen on looking from the edge 

 of a precipice into an abyss filled with fog. But the precipice, 

 though useful, is not necessary, as the following observation 

 shows. Standing upon the common, with a perfectly level 

 area of dark heather in front of us, and watching with the 

 * Communicated by the Author. 



