Cause of Iridescence in Clouds. 423 



was greatly struck both by the splendour and the frequency 

 of this phenomenon, and I availed myself of the opportunity 

 of watching the general character of the display, and securing 

 a number of measurements of the angular distances of the 

 various colours from the sun. I was thus led to an explana- 

 tion totally different to that advanced by Dr. Stoney. He 

 considers that we have here an example of the colours of thin 

 plates, while I believe the colours to originate in diffraction 

 by fine filaments of ice. Spider-threads, streaming from the 

 trees in full sunshine, often glow with most brilliant hues. 

 Replace the spider-threads by similar filaments of ice, in- 

 crease their number to billions, carry them to the distance of 

 a mile, and we should, under favourable circumstances, have 

 a cloud shining with all the colours of the spectrum. 



In the following paper I shall compare the results of each 

 theory with my observations, and show that, in the light of 

 this fuller series of facts, the theory of thin plates proves 

 unsatisfactory, while diffraction appears competent to explain 

 the whole of the phenomena. It may be that there are other 

 kinds of iridescence, which I have not been fortunate enough 

 to witness, and for which the theory of diffraction fails. But 

 even so, I shall have done good service in this inquiry by 

 pointing out both the province and the limitations of diffrac- 

 tion. Before discussing the two theories, I will give an 

 account of the phenomena I have observed, so that my 

 readers may understand precisely what the appearances are 

 that I shall attempt to explain. 



During the winter in the Engadine, the colours are fre- 

 quently brilliant enough to attract the notice of the most 

 indifferent, and on almost any day, when there are broken 

 clouds near the sun, they can be made out with the aid of 

 dark grey spectacles. The action of the spectacles depends 

 on the physiological fact, that in intense light the eye loses, 

 to a considerable extent, its power of distinguishing colour. 



Within a circle round the sun, radius about 2°, the clouds 

 are white, or faintly tinged with blue. This circular space is 

 surrounded by a ring of jeWow, passing into orange. The 

 region of most vivid hues is comprised between 3° and 7°, 

 the most striking being purple, blue, orange, green, and red. 

 These are generally scattered at random over the thinner 

 parts of the clouds. Further out, the only colours visible are 

 green and pink, becoming rapidly fainter with increasino- 

 distance. These greens and pinks are often arranged in 

 bands parallel to the edge of a cloud, sometimes as many as 

 three bands of each being visible. Broad alternate bands of 

 colour are easy to detect, even when very faint, and after 

 some practice I have thus distinguished colour as far from the 



