An X-radiograph is produced on a photographic film by X-rays transmitted through an object. In medical uses X-rays are absorbed by bones. In paintings, heavy metal pigments are opaque and appear light on the X-ray film. The X-radiographs are made by placing an X-ray tube behind or underneath a painting. The photographic film is placed over the painting and the film exposed. A negative is then developed and fixed.
X-radiographs will show obscured paint layers and changes of design that an artist has made. Millais did not re-use this canvas or make any major compositional changes. He carefully planned his work. In the X-ray below on the far left, we can see that the X-rays have been absorbed by the dense metal tacks used to secure the canvas to the stretcher. We can also see staples used to re-attach the canvas to the stretcher after the canvas was taken off for relining.
The lead white areas of the painting and the canvas weave are clearly visible in the images below as are the re-touchings that appear as black marks.