A detailed study of the colours shown during the oxidation of activated copper films supported on china clay* has shown that the primary cause of the colour sequence produced is the interference of the incident light in a thin layer of the highly refracting oxide covering the individual grains composing the film. There are, however, a number of secondary factors which produce modification in the colour sequences. These are the dispersion of the metallic oxides, their specific absorption, and the variation in the intensity of the light absorbed by the metallic reflector at the back of the film with the wave-length of the incident light. The growth of the oxide films was studied on thin films of iron, nickel, and copper supported on china clay rods, and also on massive rods of these metals, in order to throw light on the part played by these secondary factors. Comparative spectrophotometric observations were made of the character of the reflected light at frequent intervals during the oxidation. The apparatus used has already been described ( loc . cit ., pp. 575-576). The Production of Thin Activated Films of Nickel and Iron on China Clay . Cupric oxide films may be strongly ignited yet continue to adhere to their supports. Strong ignition of an iron or nickel film results in its destruction, since the oxide forms large and rather hard crystals, which brush off the rods very easily.