An X-ray is directed at the skull. Its energy is largely absorbed when it meets the dense layers of calcium. Whatever is left traverses brain tissue almost without resistance and is then once again decimated by the other side of the skull. If a morsel of energy is left at all, it makes virtually no imprint on the photographic plate: a blank spot. All X-rays together produce a negative outline of the skull. Though the brain has been traversed, it does not contribute to the image.