produced by the action of Light upon Bodies. 529 



from the exciting light, and gleam for a longer time^ at lower than 

 at higher temperatures. 



The following is a literal translation of the conclusions which 

 M. Becquered deduces from the researches recorded in his 

 memoir : — 



1. Certain bodies after being acted upon by light, and parti- 

 cularly by the more refrangible rays of the same, emit luminous 

 rays whose wave-length is in general greater than that of the 

 exciting rays. For the first few seconds this emitted light dimi- 

 nishes rapidly in intensity, and afterwards more slowly, for a time 

 which varies with different bodies from a small fraction of a 

 second up to several hours. Ultimately the bodies become com- 

 pletely inactive, and require to be exposed anew to the light in 

 order that the effect may be reproduced. 



This fact may be otherwise expressed thus : bodies manifest 

 for a certain time a persistence in the impression exerted upon 

 them by light, which depends upon the nature and physical 

 constitution of the body. This emission of light corresponds to 

 a certain totality of action received by each bodj^, and takes place 

 in obscurity, whether the body be enclosed or not. Heat merely 

 accelerates the emission of light, which at a higher tempera- 

 ture takes place in a shorter time, and consequently with a 

 greater intensity. 



2. It has been possible to construct an apparatus, to which I 

 have given the name phosphor oscope, in which the bodies can be 

 seen by an observer, and the time which intervenes between the 

 moment of insolation and that of observation can be rendered as 

 short as required, and also measured. In this apparatus a cer- 

 tain number of bodies become luminous which are not percep- 

 tibly so by the ordinary method of experiment. 



3. The refrangibility of the light emitted by phosphorescent 

 substances depends upon their molecular condition, and not 

 merely upon their chemical constitution; by employing the sul- 

 phurets of the alkaline earths, it is possible with one and the 

 same body to obtain an emission of light of different tints, and 

 that according to the temperature to which the bodies have 

 been previously submitted, and also according to the conditions 

 of the combinations which, by their reaction, give rise to the 

 substance whose phosphorescence is under consideration. 



4. In general, in the preparation of the sulphurets of the 

 alkaline earths, called artificial phosphors, the elevation of 

 temperature and the duration of tlie thermal action cause little 

 variation in the refrangibility of the light emitted by phos- 

 phorescence ; for the most ))art they only affect the luminous in- 

 tensity of these phospliors. Amongst the few bodies which 

 constitute an exception to this rule, may be mentioned the pro- 



Fhil Mag. S. 4. No. 123. fiuppl Vol. 18. 2 M 



