ABSTRACT
Prisms and lenses, whether of glass, ebonite, or aluminium, fail to afford evidence of refractive action, hence the X rays cannot be focused like those of ordinary light, and that is why the photograph of the vacuum tube had to be taken by a pin-hole. As to the nature of the rays themselves, Dr. Roentgen rejects the notion of their being "ultra-violet" rays, which was suggested by some. In consequence of ultra-violet, rays acting on the photographic plate, it is possible, as has long been known, to take a photograph in the dark. Thus, for example, if a skiagraph of the hand be attempted with an apparatus in which these factors are carried to too high a degree, the resulting X rays will pass through the bones almost as freely as through the surrounding tissues, and their shadows will therefore not appear.
