It is known that “when a thin transparent film is backed by a perfect reflector, no colours should be visible, all the light being ultimately reflected, whatever the wave-length may be. The experiment may be tried with a thin layer of gelatine on a polished silver plate.” An apparent exception has been described by R. W. Wood: “A thin film of collodion deposited on a bright surface of silver shows brilliant colours in reflected light. It, moreover, scatters light of a colour complementary to the colour of the directly reflected light. This is apparently due to the fact that the collodion film “frills,” the mesh, however, being so small that it can be detected only with the highest powers of the microscope. Commercial ether and collodion should be used. If chemically pure ether obtained by distillation is used, the filmdoes not frill, and no trace of colour is exhibited. Still more remarkable is the fact that if sunlight be thrown down upon the plate at normal incidence, brilliant colours are seen at grazing emergence, if a Nicol prism is held before the eye. These colours change to the complementary tints if the Nicol is rotated through 90°, i.e. in the scattered light, one half of the spectrum is polarized in one plane, and the remainder in a plane perpendicular to it.”

I have lately come across an entirely forgotten letter from Rowland in which he describes a similar observation. Writing to me in March 1893, he says:—“While one of my students was working with light reflected from a metal, it occurred to me to try a thin collodion film on the metal.