162 P^of. Faraday i on Phosphorescence^ ^c. [June 17, 



ments this cylinder could be revolved, and the part which was at one 

 instant within, rapidly brought to the outside, and observed by the 

 audience. As the cylinder could be made to revolve 300 times in a 

 second, and as the twentieth part of a revolution was enough to bring a 

 sufficient portion of the cylinder to the outside, it is evident that a 

 phosphorescent effect which would last only the 1 -3000th or even the 

 1 -6000th of a second might be made apparent. All escape of light 

 between the moving cylinder and the box was prevented by the use of 

 properly attached black velvet. 



The cylinder was first supplied with a surface of Becquerel's 

 phosphori. The eifect here was, that when by rotation the part 

 illuminated was brought outside the box it was found phosphorescent. 

 If the cylinder continued to rotate it appeared equally luminous all 

 over, and when the rotation ceased, or the lamp was extinguished, the 

 light gradually sank as the phosphorescence fell. Then a cylinder 

 having a surface of quinine or sesculin was put into the apparatus. 

 Whilst the cylinder was still it was dark outside ; but when revolving 

 with moderate velocity it became luminous outside, ceasing to be so 

 the moment the revolution stopped. Here the fluorescence was evi- 

 dently shown to occupy time ; indeed, the full time of a revolution : 

 and taking advantage of that, the self-shining of the body was separated 

 from its illumination within, and the fluorescence made to assume the 

 character of phosphorescence. Another cylinder was covered with 

 crystals of nitrate of uranium, a hot saturated solution having been 

 applied over it with a fine brush. The result was beautiful. A 

 moderate degree of revolution brought no light out of the box ; but 

 with increased motion it began to appear at the edge. As the rapidity 

 became greater, the light spread over the cylinder, but it could not be 

 carried over the whole of its surface. It issued as a band of light 

 where the moving cylinder left the edge of the box, diminishing 

 in intensity as it went on, and looking like a bright flame, wrapping 

 round half the cylinder. When the direction of revolution was re- 

 versed, this flame issued from the other side ; and when the motion of 

 the cylinder was stopped, all the phenomena of fluorescence or phospho- 

 rescence disappeared at once. The wonderfully rapid manner in which 

 the nitrate of uranium received the action of the light within the box, 

 and threw off its phosphorescence outside, was beautifully shown. 



The electric light, even when the discharge is in rarefied media, or 

 as a feeble brush, emits a great abundance of those rays, which produce 

 the phenomena of fluorescence ; but then if these rays have to pass 

 through common glass they are cut off, being absorbed and destroyed 

 even when they are not expended in producing fluorescence or phos- 

 phorescence. Arrangements can however be made in which the 

 advantageous circumstances can be turned to good account with 

 such bodies as Becquerel's phosphori or uranium glass. If these be 

 enclosed within glass tubes, having platinum wires at the extremities, 

 and which are also exhausted of air and hermetically sealed, then the 

 discharges of a RuhmkorfF coil can be continually sent over the phos- 

 phori, and the effects both fluorescent and phosphorescent be beauti- 



