642 Sroney—On the Cause of Iridescence in Clouds. 
why cirrus clouds are occasionally iridescent; but the rest of the account of this 
accurate observer, and the admirable drawings which he made of the crystals, will 
also well repay careful study. Thus, if the cloud consist of crystals of Scoresby’s 
fifth genus, or if crystals of the first and third genera are both present, and if the 
air is so calm that the crystals can remain in the terminal position into which they 
would come in falling through still air, then we shall have the phenomenon of 
both a horizontal and a vertical circle through the sun making a cross. Whereas, 
if the crystals are of the first genus only, the vertical circle will present itself 
without the horizontal. I have myself seen the phenomenon in this latter form. 
Crystals of either the first or the fifth genus, if occasionally agitated so that they 
will keep fluttering, would give rise to iridescence if of sufficiently uniform 
thickness. 
Captain Scoresby describes lamellar flakes of snow floating in the air and 
sparkling in the sunbeams, as always present during severe frosts, when the sky is 
clear. ‘The beautiful appearance they would have is a familiar one in chemical 
laboratories, when a glass vessel, in which precipitated tabular crystals are sub- 
siding through the mother liquid, is placed in the direct light of the sun. The 
whole liquid then seems alive with minute specks flashing with the brilliant 
colours of thin plates. 
