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LXXVII. Remarks upon an Optical Phenomenon, seen in Swit- 

 zerland. By H. F. Talbot, Esq. M.P. FM.S* 



TN the Number of the Philosophical Magazine for November 

 ■*• (page 332) is an account, by Professor Necker, of a pretty 

 optical phenomenon, seen in Switzerland, when the sun rises 

 from behind distant trees standing on the summit of a moun- 

 tain. The Professor adds, that he is surprised it has never 

 been noticed before. 



I am happy to be able to bear testimony to the accuracy of 

 his description of this phenomenon, having observed it myself 

 with great attention in the summer of 1823, with the view of 

 discovering its cause. The place of my observation was in the 

 Val Levantine, at the foot of a cliff about a thousand feet high, 

 whose summit was fringed with pine trees : — it was about the 

 middle of the day, and the sun was very elevated. By ap- 

 proaching the cliff, or retiring from it, I could make its sum- 

 mit conceal the sun's disk or not, at pleasure, so that I could 

 observe the appearances which took place with great facility. 

 I observed with a telescope, of which Mr. Necker makes no 

 mention ; but I think it is absolutely necessary, in order to see 

 the full beauty of the phenomenon. 



When the sun is about to emerge from behind the crest of 

 pine trees on the cliff's summit, every branch and leaf is lighted 

 up with a silvery lustre of indescribable beauty. But it will be 

 seen, by observing the trunks of the trees and the larger 

 branches, that this silvery light forms only a margin to every 

 object : it is only their outline which is luminous. Of course 

 this cannot be discerned with respect to the smaller and more 

 complicated objects (such as the foliage), which therefore ap- 

 pear altogether luminous. The birds, as Mr. Necker very truly 

 describes, appear like flying brilliant sparks : others are seen 

 occasionally, smaller than these, which may in all probability 

 be insects or butterflies. 



With regard to the cause of this appearance, I have no he- 

 sitation in ascribing it to diffraction. It may be seen not only 

 in Switzerland, but to a certain extent in any country, by ob- 

 serving with a telescope a distant building from behind which 

 the moon is going to emerge. Just before the emersion the 

 outline of the building acquires a bright silvery appearance. 

 But the Swiss phenomenon, which is a very striking one, can- 

 not be well observed except in a mountainous country. 



Communicated by the Author. 



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