A remarkable phosphorescent property was discovered by M. E. Becquerel in the solar spectrum. Two luminous bands separated by a dark one are excited by the solar spectrum on paper covered with a solution of gum arabic, and strewed with powdered sulphuret of calcium or Canton’s phosphorus. One of the luminous bands occupies the space under the least refrangible violet rays, and the other that beyond the lavender rays, so that the dark band lies under the extreme violet and lavender rays. When the action of the light is continued, the whole surface beyond the least refrangible violet shines, the luminous bands already mentioned brightest; but all the space from the least refrangible violet to the extreme red remains dark. If the surface, prepared either with the sulphuret of calcium or Bologna stone, be exposed to the sun’s light for a little time, it becomes luminous all over; but when, in this state, a solar spectrum is thrown upon it, the whole remains luminous except the part from the least refrangible violet to the extreme red, on which space the light is extinguished; and when the temperature of the surface is raised by a lamp, the bright parts become more luminous and the dark parts remain dark. Glass stained by the protoxide of copper, which transmits only the red and orange rays, has the same effect with the less refrangible part of the spectrum; hence there can be no doubt that the most refrangible and obscure rays of the spectrum excite phosphorescence, while all the less refrangible rays of light and heat extinguish it.
Paper prepared with the sulphuret of barium, when under the solar spectrum, shows only one space of maximum luminous intensity, and the destroying rays are the same as in the sulphuret of calcium. Thus the obscure rays beyond the extreme violet produce light, while the luminous rays extinguish it.
The phosphoric spectrum has inactive lines which coincide with those in the luminous and chemical spectra, at least as far as it extends; but in order to be seen the spectrum must be received for a few seconds upon the prepared surface through an aperture in a dark room, then the aperture must be closed, and the temperature of the surface raised two or three hundred degrees; the phosphorescent parts then shine brilliantly and the dark lines appear black. Since the parts of similar refrangibility in different spectra are traversed by the same dark lines, rays of the same refrangibility are probably absorbed at the same time by the different media through which they pass.