Door of W.C. Röntgens lab.

In his experiments, Röntgen also took pictures through his laboratory door, which allowed a passage to the neighboring room. He could detect streaks on the photo plates in the other room. Röntgen later said: "I noticed this shadowing and realized that it was not the absorption due to the uneven wood thicknesses of the door jamb that was decisive, but the surface absorption of the post. I inquired about the style of the door paint and learned that it was lead white. Because lead is so heavily permeable to these rays, a lead white layer running in the direction of the rays absorbs considerably more than a layer oriented perpendicular to the rays."