of Iridescence in Clouds. 89 



on the parallel surface behind, a portion will be there reflected, 

 and, passing out through the front face, will also reach the 

 eye of the observer. These two portions of the ray — that 

 reflected from the front face and that reflected from the back 

 — are precisely in the condition in which they can interfere 

 with one another, so as to produce the splendid colours with 

 which we are familiar in soap-bubbles. If the crystals are of 

 diverse thicknesses the colours from the individual crystals 

 will be different, and the mixture of them all will produce 

 merely white light ; but if all are nearly of the same thick- 

 ness, they will transmit the same colour towards the observer, 

 who will accordingly see this colour in the part of the cloud 

 occupied by these crystals. The colour will, of course, not 

 be undiluted ; for other crystals will send forward white light, 

 and this, blended with the coloured light, will produce delicate 

 shades in cases where the corresponding colours of a soap- 

 bubble would be vivid. 



We have now only to explain how it happens that on very 

 rare occasions the colours, instead of lying in irregular patches, 

 form definite fringes round the borders of the cloudlets. The 

 circumstances that give rise to this special form of the phe- 

 nomenon appear to be the following : — While the cloud is in 

 the process of growth (that is, so long as the precipitation of 

 vapour into the crystalline state continues to take place) so 

 long will the crystals keep augmenting. If, then, a cloudlet 

 is in the process of formation, not only by the springing up of 

 fresh crystals around, but also by the continued growth of the 

 crystals within it, then will that patch of cloud consist of 

 cr} r stals which are largest in its central part, and gradually 

 smaller as their situation approaches the outside. Here, then, 

 are conditions which will produce one colour round the margin 

 of the cloud, and that colour mixed with others, and so giving 

 rise to other tints, further in. In this w T ay there comes into 

 existence that iris-like border which is now and then seen. 



The occasional upsetting of the crystals, which is required 

 to keep them fluttering, may be produced in any of three 

 ways. The cloudlets may have been formed from the blending 

 together of two layers of air saturated at different temperatures, 

 and moving with different velocities or in different directions. 

 Where these currents intermix a certain amount of disturbance 

 will prevail, which, if sufficiently slight,, would not much 

 interfere with the regularity of the crystals, and might yet be 

 sufficient to occasion little draughts, which would blow them 

 about when formed. Or, if the colder layer is above, and if 

 it is in a sufficient degree colder, there need not be any pre- 

 vious relative motion of the two layers ; the inevitable con- 



