1859.] Prof. Faraday on Phosphorescence, &;c. 159 



WEEKLY EVENING MEETING, 



Friday, June 17, 1859. 



The Lord Wensleydale, Vice-President, in the Chair. 



Professor Faraday, D.C.L. F.R.S. 

 On Phosphorescence, Fltiorescence, Sfc, 



The agent understood by the word "light," presents phenomena so 

 varied in kind, and is excited to sensible action by such different 

 causes, acting apparently by methods differing greatly in their physical 

 nature, that it excites the hopes of the philosopher much in relation to 

 the connexion which exists between all the physical forces, and the 

 expectation that that connexion may be greatly developed by its means. 

 This consideration, with the great advance in the experimental part 

 of the subject which has recently been made by E. Becquerel, were 

 the determining causes of the production of this subject before the 

 members of the Royal Institution on the present occasion. 



The well known effect of light in radiating from a centre, and ren- 

 dering bodies visible which are not so of themselves, as long as the 

 emission of rays was continual — the general nature of the undulatory 

 view, and the fact that the mathematical theory of these assumed 

 undulations was the same with that of the undulation of sound, and of 

 any undulations occurring in elastic bodies, were referred to as a starting 

 position. Limited to this effect of light it was observed that the 

 illuminated body was luminous only whilst receiving the rays or un- 

 dulations. 



But superadded occasionally to this effect is one known as phos- 

 phoresce??.ce, which is especially evident when the sun is employed as the 

 source of light. Thus, if a calcined oyster -shell, a piece of white paper, 

 or even the hand, be exposed to the sun's rays and then instantly placed 

 before the eyes in a perfectly dark room, they are seen to be visible 

 after the light has ceased to fall on them. There is a further philosophi- 

 cal difference, which may be thus stated ; if a piece of white oyster- 

 shell be placed in the spectrum rays issuing from a prism, the parts 

 will, as to illumination, appear red, or green, or blue, as they come 

 under the red, green, or blue rays : whereas if the phosphorescent 

 effect be observed, i.e. that effect remaining after the illuminating rays 

 are gone, the light will either be white, or of a tint not depending 

 upon the colour of the ray producing it, but upon the nature of the 

 substance itself, and the same for all the rays. 



The ray which comes to the eye in an ordinary case of visibility, 

 may be considered as that which, emanating from the luminous body, 



