The brightness of that point on the film is the summation of all the densities the ray encountered. A ray that passes through bone and tissue records a brighter image than a ray that passes through just bone. A ray that grazes the edge of the skull travels over a large distance of bone, which makes that part of the image very bright. A ray that traverses thin bone, such as the area around the temples on a lateral X-ray, records a darker gray image. A normal tissue in the neck may show up as a particular shade of gray.
Density is a function of mass and volume.
Increasing the volume lowers the density. Thus, this tissue will show up as a darker shade of gray than the normal tissue. A radiologist needs to see the full range of densities, that is, shades of gray from white to black, to make a proper diagnosis. Some X-rays can have black areas that are over 500 times darker than the brightest areas, although the typical range in the tissues of interest is much smaller than this, about 32 to 1. X-rays are transparent like photographic slides rather than prints.