Although ultraviolet fluorescence, infrared photography and x-radiography are useful in studying many different art materials, the author limits his answer to the examination of a late 15th century Italian painting. The fact that each device is deceptive because of what it does not show prompts the realization that the three complement each other and should be used with constant reference to the object. An infrared photograph of a repainted area shows that in spite of a color match in visible light, the new paint may reflect only a little infrared energy compared to the old paint, resulting in a dark line on the photograph. If it reflects much infrared radiation the photographic image will be light in tone. A photograph of ultra violet fluorescence records the uppermost layers of an object, the varnish or paint directly under it. If the photograph shows dark interruptions in the visible yellow green fluorescence of varnish, these may be new paint on top of the varnish. Other systems of retouching under the varnish may be fainter in tone. In the painting studied, x-rays provided important information about the various densities of the ma terials used, also the fact that the picture had been transferred, itsoriginal support having been replaced by a new, but aged panel. Experience in interpreting the evidence is emphasized.