The coefficients of transmission and reflection of thin aluminium films deposited on silica by vacuum evaporation were measured for wavelengths between 4200 and 2600Å. The results were applied to the calculation of the width and intensity of Fabry - Perot fringes obtainable with aluminium coated plates. Measurements of widths of heterochromatic fringes gave upper limits of the reflectivity at wavelengths of 2400Å and below, and showed that 1/18 of one order can be resolved in this range without excessive loss of intensity if the thickness of the films is properly chosen.