In a paper “On the Phenomena of Thin Plates of Solid and Fluid Substances exposed to Polarised Light,” published in the “Philosophical Transactions” for 1841, I had occasion to notice certain motions and colours which I had observed upon films of some of the volatile oils; but as they were unconnected with the subject I was then investigating, I made no attempt to discover their nature and origin. Their apparent similarity, however, to the molecular movements and colours upon the soap-bubble, induced me to resume the subject, and to examine them as exhibited upon films of various evaporable liquids, stretched over apertures differing in size, form, and substance.