In "Mathematics. Queen & Servant of Science" E. T. Bell mentions the
effect of conical refraction for certain crystals was predicted by
Hamilton (a good source of his writings on this topic may be found on http://www.maths.tcd.ie/pub/HistMath/People/Hamilton/ConicalRefraction/)
and later verified experimentally. Are there any good more recent
references on this topic ? Is the number of possible refracted rays
determined by the number of nonequivalent (due to asymmetry) principal
planes, so that the only possible number of refrated rays from an
incident ray is 1, 2, 3 or infinite ?
Thanks in advance.
Inge H. A. Pettersen