LIGHT. 125 



that the eye cannot detect its duration. M. E. Becquerel 

 has made many experiments which support this view ; the 

 fact of the phosphorescence by insolation of a large number 

 of bodies, is in itself evidence of the matter of which they are 

 composed being thrown into a state of undulation, or at all 

 events molecularly affected by the impact of light, and is 

 therefore an argument in support of the view to which objec- 

 tion is taken. Dr. Young admits that the phenomena of 

 solar phosphorus appear to resemble greatly the sympathetic 

 sounds of musical instruments, which are agitated by other 

 sounds conveyed to them through the air, and I am not aware 

 that he gives any explanation of these effects on the ethereal 

 hypothesis. 



Some curious experiments of M. Niepce de St. Victor 

 seem also to present an analogy in luminous phenomena to 

 sympathetic sounds. An engraving which has been kept for 

 some days in the dark is half covered by an opaque screen, 

 and then exposed to the sun ; it is then removed from the 

 light, the screen taken away, and the engraving placed oppo- 

 site, and at a short distance from, photographic paper : an 

 inverted image of that portion of the engraving which has 

 been exposed to the sun is produced on the photographic 

 paper, while the part which had been covered by the screen 

 is not reproduced. If the engraving, after exposure, is 

 allowed to remain in contact with white paper for some hours, 

 and the white paper is then placed upon photographic paper, 

 a' faint image of the exposed portion of the engraving is repro- 

 duced. Similar results are produced by mottled marble ex- 

 posed to the sun ; an invisible tracing on paper by a fluores- 

 cent body, sulphate of quinine, is, after insolation, reproduced 

 on the photographic paper. Insolated paper retains the power 

 of producing an impression for a very long period, if it is kept 

 in an opaque tube hermetically closed. 



It is right to observe that these effects are supposed by 

 many to be due to chemical emanations proceeding from the 



