Described by Dr. Knott as "rays of light that pass with ease through many substances that are optically opaque, but are absorbed by others." "For example," he says, "the bony structures of the body are much less transparent than the fleshy parts; hence by placing the hand between a fluorescent screen and the source of these rays we see the shadow of the skeleton of the hand with a much fainter shadow of the flesh and skin bordering it." See Dr. Knott's "Physics."