Stoney—On the Cause of Iridescence in Clouds. 639 
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 crystals 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, farther in. In this way 
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 previous relative motion of the two layers; 
the inevitable convexion currents will suffice. Another, and probably the most 
frequent, cause for little breezes in the neighbourhood of the cloudlets is, that when 
the cloudlets are formed they immediately absorb the heat of the sun in a way that 
the previously clear air had not done. If they absorb enough they will rise like 
feeble balloons, and slight return currents will travel downwards round their 
margins, throwing all crystals in that situation into disorder. 
I do not include among the causes which may agitate the crystals another cause 
which must produce excessively slight currents of air, namely, that arising from 
the subsidence of the cloudlets owing to their weight. The crystals will fall faster 
where in cloud masses than in the intervening portions where the cloud is thinner. 
But the subsidence itself is so slow, that any relative motions to which differ- 
ences in the rate of subsidence can give rise are probably too feeble to produce an 
appreciable effect. Of course, in general, more than one of the above causes will 
concur, and it is the resultant of the effects which they would have separately that 
will be felt by the crystals. 
If the precipitation had taken place so very evenly over the sky that there were 
no cloudlets formed, but only one uniform veil of haze, then the currents which 
would flutter the crystals may be'so entirely absent that the little plates of crystal 
can fixedly assume the horizontal position which is natural to them. In this event 
the cloud will exhibit no iridescence, but instead of it a vertical circle through the 
sun will present itself. This on some rare occasions isa feature of the phenomenon 
of parhelia. 
It thus appears that the occasional iridescence of cirrus clouds is satisfactorily 
accounted for by the concurrence of conditions, each of which is known to have a 
