Researching iridule. I got it in my head that most readers interpreted iridules as sundogs, also called parhelion. Google somewhat surprisingly returned a Fri, 19 Jun 2009 22:00:42 -0400 post from the archives from Jerry Friedman:
Nabokov, apparently. Brian Boyd says that in the summer of 1951, in
Telluride, Colorado, the Nabokovs saw a rainbow every evening and often
saw what VN would call an iridule in /Pale Fire/ (VNAY, pp. 201-202).
Unfortunately for us fans of atmospheric optical phenomena, Brian
doesn't describe what the Nabokovs said, tell us whether Nabokov said at
the time that the "iridule" was a reflection of the rainbow, or give his
own thoughts or expert comments about any interpretation Nabokov may
have made. His source may be a letter from Véra to Igor Trofimov.
As I've said here before, I don't think Shade's description can be
correct, because I don't believe a cloud can reflect an image,
especially of something distant and no brighter than its surroundings.
Kinbote's comments add to the confusion. An iridescent cloud is one
thing, a parhelion is another, and a mother-of-pearl cloud is yet
another.
http://en.wikipedia.org/wiki/Cloud_iridescence
http://en.wikipedia.org/wiki/Parhelion
http://en.wikipedia.org/wiki/Mother-of-pearl_cloud
I still suspect that Nabokov saw a parhelion and a rainbow at the same
time and thought, or let Shade think, that the former was a reflection
of the latter. In that case Kinbote, of all people, would be right.
from Wikipedia:
Cloud iridescence is the occurrence of colors in a
cloud not dissimilar to those seen in oil films on puddles, and is similar to
irisation. It is a fairly uncommon phenomenon and is usually observed in
altocumulus,
cirrocumulus and
lenticular cloudsbut very rarely in
Cirrus clouds.
[1][2][3] The colors are usually pastel but sometimes they can be very vivid. Iridescence is most frequently seen near to the sun with the sun's glare masking it. It is most easily seen by hiding the sun behind a tree or building. Other aids are dark glasses or observing the sky by its reflection in a convex mirror or in a pool of water.
Iridescent clouds are a
diffraction phenomenon. Small
water droplets or even small
ice crystals in clouds individually scatter light. Large ice crystals produce
halos, which are refraction phenomena rather than iridescence. Iridescence should similarly be distinguished from the refraction in larger raindrops that gives a
rainbow.
If parts of the clouds have droplets (or crystals) of similar size the cumulative effect is seen as colors. The cloud must be optically thin so that most rays encounter only a single droplet. Iridescence is therefore mostly seen at cloud edges or in semi-transparent clouds. Newly forming clouds produce the brightest and most colorful iridescence because their droplets are of the same size. When a thin cloud has droplets of similar size over a large extent the iridescence takes on a structured form to give a
corona, a central bright disk around the sun or moon surrounded by one or more colored rings.
–––––––––––––––––––––––––––––––––
I greatly appreciate the references, but I disagree with the conclusion. I think the phenomenon is simply cloud iridescence. My previous thought was it was a hoax that pointed to how much the reader is willing to believe the words of a writer or one of his character. I was pretty certain it wasn't a sundog, they're too different. But shimmering pastels sounds rather rare and wonderful to me, and would be something likely to be remembered.
The definition that Shade gives is to be taken metronymic-ally: serving merely to associate iridule and rainbow, the container and the contained.