The colours of the soap-bubble have been the subject of frequent observation since the time of Boyle, Hook, and Newton, and they have been invariably ascribed “not to any colour in the medium itself in which they are formed, or on whose surfaces they appear, but solely to its greater or less thickness.” The author of this paper had been led to doubt the correctness of this opinion, and while repeating the beautiful experiments of Professor Plateau “On the equilibrium of liquid films,” he was led to discover the true cause of these colours, whether they are observed on the soap-bubble or on plane, convex, and concave films stretched across the mouths of closed or open vessels.