LIGHT FROM THE EDGE OF THE SUN. 185 



paper, and expose it to the action of the solar spectrum, 

 it is found to glow (in the dark) only over those spaces 

 occupied by the violet rays and the ordinarily dark rays 

 beyond them ; proving that the excitation necessary to 

 the development of the phenomena of phosphorescence 

 is due to a class of rays distinct from the true light- 

 giving principle, and more nearly allied to that principle 

 or power which sets up chemical decomposition. 

 Whether the fluorescent rays, before mentioned, which 

 are found so abundantly over the space which produces 

 the greatest phosphorescent effect, are active in pro- 

 ducing the phenomena, is as yet an unsolved problem. 



Vision and colour, calorific action, chemical change, 

 molecular disturbance, electrical phenomena, and phos- 

 phorescent excitation, all, each one with a strange 

 duality, are connected with the sunbeam. 



We find, when we receive solar spectra upon iodized 

 plates, or on several kinds of photographic paper, that a 

 line, over which no action takes plates, is preserved at 

 the top and bottom of the impressed image, and in many 

 cases along the sides also. The only way in which this 

 can be accounted for, as the spectrum represents the 

 sun in a distorted form, is by supposing that rays come 

 from the edges of the sun of a different character from 

 those which proceed from the centre of that orb.* 



Light from the centre of the solar disc is under dif- 

 ferent conditions from that which comes from the edge 

 of the sun : this is due to the varying angle, which is 

 present to us by a circular body : calorific action seems 



' : The chemical evidence of this will be found in Sir John 

 Herscbel's Memoir On the Solar Spectrum, and particularly as 

 exemplified in the changes produced on the tartrate of silver. 

 Similar influences are described as observed on a Daguerreotype 

 plate, in a paper entitled Experiments and Observations on Light 

 which has permeated coloured media, and on the Chemical Action of 

 the Solar Spectrum ; by Kobert Hunt. Philosophical Magazine, 

 vol. xxvi. 1840. 



