Concerns the measurement of color change in pictures by use of the reflectance spectro-photometer designed by Professor W. D. Wright and Dr. M. P. Wassall. A detailed description of the instrument is given together with diagrams of the optics, an outline of the calibration and recording procedures. The reflectance data is convened to C. I. E. Tristimulus values and chromaticity co-ordinates. The equations defining these quantities appear in an appendix to the article. Presents chromaticity data and reflectance curves for known stable and fugitive pigments in test paintings, before and after fading R. Wh.