216 PHOSPHORIC SPECTRUM. SECT. XXIV. 



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 sul- 

 phuret 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 men- 

 tioned 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 re- 

 frangible 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 in- 

 tensity, 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 aperature 

 in a dark room, then the aperture must be closed, and the tem- 

 perature 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 dif- 

 ferent media through which they pass. 



