Cause of Iridescence in Clouds. 433 



purple. Out of some thirty or forty measurements of this 

 colour, taken at various times and places, I have never found 

 a case within 3°, or without 7°. The first gives a diameter 

 *019 millim., the second *0085 millim. So, whatever may be 

 the cause, we are driven to conclude that filaments outside 

 these limits are not found in the clouds in sufficient numbers 

 and with sufficient uniformity of size to produce the brighter 

 iridescences. (Coronae, mentioned below, are seen in quite a 

 different type of cloud. The colours form regular circles, 

 and are of a different character, being dull and blurred.) 



Dr. Stoney's description of the phenomena agrees, as far as 

 it goes, fairly well with mine, though I have not noticed that 

 iridescence is more frequent when the sun is low. Indeed, 

 it has seemed to me to be less frequent, owing to the greater 

 thickness of the cloud-layer, measured in the direction of the 

 sun. But near the horizon the colours, of course, are more 

 likely to catch the eye. 



If he is correct in supposing that the laminae would generally 

 be but slightly inclined to the horizontal, the thin-plate colours 

 would be seen only above and below the sun, and not at either 

 side. I have never seen any sign of such a tendency. 



He says : — " Some few times in one's life the display may 

 be seen in all quarters of the sky, and with the sun well up 

 in the heavens." If the phrase " in all quarters of the sky " 

 is to be taken literally, the phenomenon is quite different from 

 anything I have seen, and can, indeed, be scarcely explained 

 on my hypothesis. 



Another phenomenon, nearly related to iridescence, is that 

 of coronae. I have seen these in the winter at St. Moritz, 

 when the sky was covered with a white haze, gathered here 

 and there into more definite wisps. The colours were much 

 fainter, and were arranged in regular rings. I have distin- 

 guished two complete spectra, the size of the rings pointing 

 to filaments about 'OI millim. in diameter. 



Both coronae and iridescence are also produced by water- 

 drops, and I have calculated the brightness of the cloud in 

 this case also. The results for the first, second, and fourth 

 spectra are : — 



•0071a/f 2 , 



•0046*Af 

 •0027«/f 



where yjr, as before, is expressed in degrees, and the unit of 

 brightness is that of the sun. These are only slightly greater 

 than the corresponding figures for ice-clouds, and the dif- 



