Cause of Iridescence in Clouds. 87 



is also a small error due to capacity. This suggestion is made 

 with hesitation, as it seems probable that somebody must have 

 thought of the arrangement or tried it. It is infinitely easier 

 to think of a novelty, or an apparent novelty, than to find out 

 whether it is really new. For instance, on looking the matter 

 up a little for this note, the writer found that the null method 

 of comparing the coefficients of mutual induction of coils is 

 mentioned in a footnote to Brillouin's elaborate memoir on 

 the comparison of coefficients of induction. Maxwell also 

 mentions a loaded differential galvanometer for comparing 

 frequent condenser-discharges with a known current. Roiti's 

 method is also given in Mascart and Joubert's book. 



VIII. On the Cause of Iridescence in Clouds. By Gr. Johnstone 

 Stoney, M.A., D.Sc.j F.R.S.; a Vice-President, R.B.S.* 



WHEN the sky is occupied by light cirro-cumulus cloud, 

 an optical phenomenon of the most delicate beauty 

 sometimes presents itself, in which the borders of the clouds 

 and their lighter portions are suffused with soft shades of 

 colour like those of mother-of-pearl, among which lovely pinks 

 and greens are the most conspicuous. Usually these colours 

 are distributed in irregular patches, just as in mother-of-pearl; 

 but occasionally they are seen to form round the denser 

 patches of cloud a regular coloured fringe, in which the 

 several tints are arranged in stripes following the sinuosities 

 of the outline of the cloud. 



I cannot find in any of the books an explanation of this 

 beautiful spectacle, all the more pleasing because it generally 

 presents itself in delightful summer weather. It is not men- 

 tioned in the part of Moigno's great Repertoire oV Ojptique 

 which treats of meteorological optics, nor in any other work 

 which I have consulted. It seems desirable, therefore, to 

 make an attempt to search out what appears to be its expla- 

 nation. 



At the elevation in our atmosphere at which these delicate 

 clouds are formed the temperature is too low, even in mid- 

 summer, for water to exist in the liquid state ; and, accord- 

 ingly, the attenuated vapour from which they were condensed 

 passed at once into a solid form. They consist, in fact, of 

 tiny crystals of ice, not of little drops of water. If the preci- 

 pitation has been hasty, the crystals will, though all small, be 

 of many sizes jumbled together, and in that case the beautiful 



* "Reprinted, by permission, from the Scientific Transactions of the 

 Royal Dublin Society of the 16th February and 23rd March, 1887. 



