638 Sronry—On the Cause of Iridescence in Clouds. 
his observations it appears (a) that when formed at temperatures several degrees 
below the freezing-point, the crystals, whether simple or compound, are nearly all 
of symmetrical forms; (/) that thin tabular crystals are extremely numerous, 
consisting either of simple transverse slices of the fundamental hexagon, or, more 
frequently, of aggregations of these attached edgewise and lying in one plane; 
and (¢) that, according as atmospheric conditions vary, one form of crystal or 
another largely preponderates. A fuller account of these most significant observa- 
tions is given in the Appendix to this Paper. 
Let us then consider the crystals in any one neighbourhood in the sky, where 
the conditions that prevail are such as to produce lamellar crystals of nearly the same 
thickness. The tabular plates are subsiding through the atmosphere—in fact 
falling towards the earth. And although their descent is very slow, owing to their 
minute size, the resistance of the air will act upon them as it does upon a falling 
feather: it will cause them, if disturbed, to oscillate before they settle into that 
horizontal position which flat plates finally assume when falling through quiescent 
air. We shall presently consider what the conditions must be, in order that the 
crystals may be liable to be now and then disturbed from the horizontal 
position. If this occasionally happens, the crystals will keep fluttering, and at 
any one moment some of them will be turned so as to reflect a ray from the 
sun to the eye of the observer from the flat surface of the crystal which 
is next him. Now, if the conditions are such as to produce crystals which 
are plates with parallel faces, and as they are also transparent, part only of the 
sun’s ray that reaches the front face of the crystal will be reflected from it: the 
rest will enter the crystal, and, falling 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. Ifthe 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 thickness, 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 phenomenon 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 
