The electric light is particularly rich in these invisible rays. The dark heat rays may be sifted from the beam of light by passing it through a transparent solution of alum; only the luminous rays will be allowed to pass. The luminous rays may be sifted out by sending the beam through an opaque solution of iodine in carbon di-sulphide. If these solutions be placed in spherical flasks, they will constitute lenses that will refract the transmitted rays to well-defined foci. The focus of the transparent solution will be brilliantly illuminated, but will have little heating power; that of the opaque solution will be invisible, while gun-cotton placed there may be instantly exploded. Platinum-foil has been raised to a red heat at one of these dark foci. Photographs are now frequently taken by the electric light.