Providing a transparent layer of an oxide of an element from group IVa of the Periodic Table, notably TiO.sub.2, by providing a substrate with a solution of a compound of the element which upon heating is converted into the relevant oxide, drying the film and heating the dried film so as to form the transparent layer of the oxide. The oxide thus obtained is a form having a comparatively low refractive index. By heating the product, after providing the film, rapidly to a temperature of above 700.degree. C., preferably above 1,000.degree. C., keeping it at this temperature for some time and then rapidly cooling it again, a modification having a higher refractive index (for example, for TiO.sub.2 rutile) is obtained.