produced by the action of Light upon Bodies. 527 



the principal phosphorescent sulphurets above mentioned. The 

 author gives the results of his numerous experiments on this 

 subject in the form of a Table, wherein the principal phospho- 

 rescent substances are arranged according to the duration, as 

 well as according to the intensity, of the light which they emit 

 after insolation. To facilitate his researches, M. Becquerel in- 

 vented an apparatus, the phosphoroscope, wherein bodies may be 

 regarded by the experimenter immediately (a small fraction of a 

 second) after the action of the light, so that the interval which 

 separates the moment of observation from that of luminous action 

 may be shortened to any extent, as well as measured. In this 

 apparatus a great many bodies become luminous which do not 

 appear to be so by the ordinary mode of experiment. By means 

 of a modification introduced into such an apparatus, which per- 

 mitted of its being placed in the path of solar rays refracted by 

 a prism, the author was able to verify the action of the different 

 rays of the spectrum upon several bodies. 



The action of the electric light on phosphorescent bodies is 

 next examined. It had already been observed that the electric 

 light, particularly that proceeding from the discharge of a bat- 

 tery, acts energetically in rendering sensitive substances lumi- 

 nous. In order the better to study the phaenomena of phospho- 

 rescence due to the action of the electric light, the author, by a 

 new disposition of his apparatus, applied to electricity the same 

 principle which served as a basis for the construction of the 

 phosphoroscope. With this object in view he introduced the 

 phosphorescent bodies, in fragments or in powder, into glass 

 tubes 40 or 50 milliuis. in length and 2 or 3 centims. in dia- 

 meter, from which he extracted the air up to 1 or 2 thousandths 

 of pressure. Through these tubes, held horizontally, he passed 

 an electric discharge proceeding from a machine, a battery, or a 

 Ruhmkorff^s coil. In all cases the electric rays which traverse 

 the rarefied air of the tube, and which, as is well known, present 

 the violet tints of the very refrangible rays, excite the phospho- 

 rescence of the bodies to the highest degree on passing very near 

 their surfaces. The phosphorescent bodies not only preserve 

 their luminosity for a certain time after the passage of the cur- 

 rent, as if they had been exposed to the sun's rays, but even 

 during the passage itself they remain luminous in a continuous 

 manner, and that with an intensity which increases with the 

 energy of the electric action. It will be at once seen how well 

 adapted this mode of experiment is to render perceptible the 

 action of light upon bodies where the luminous impression lasts 

 for a very sliort time, and even escapes the phosphoroscope. The 

 reader will find in the original memoir curious details on this 

 subject. 



