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. 
ADDENDUM.—JUNWE 16, 1887. 
When the fluttering of the lamellar crystals which form an iridescent cloud is 
gentle, the crystals will not incline much from a horizontal position ; and as this is 
the case which most frequently occurs, it is desirable fully to consider the conse- 
quences of it. With clouds of this kind the iridescent colours will be seen only 
when the sun is low in the sky, and only in parts of the cloud that are at no great 
distance from him, and that are rather above or below the sun than sideways. In 
more distant clouds, when the sun is low—and in all the clouds when the sun is 
high in the sky—the flat surfaces of the crystals do not become sufficiently inclined 
to reflect the sun’s rays to the spectator, and accordingly all the light which 
reaches the eye from the clouds that are so situated has been reflected by edges of 
crystals, or scattered in irregular ways, and is mere white light. This is the case 
that is oftenest seen: only a few clouds near the setting or rising sun exhibit their 
soft iridescence. But some few times in one’s life the display may be seen in all 
quarters of the sky, and with the sun well up in the heavens, and the phenomenon 
is then one of the most enchanting presented to us by nature. This exquisite 
spectacle can only present itself on those rare occasions when the crystals are 
tossed about in an unusual degree. 
S107 O07 
