472 Scientific Intelligence. [June 



VI. — Optical Experiment. 



The following is contained in a letter to the Editor, from Pro- 

 fessor Stevelly of Belfast. 



" Mr. Patterson, a young gentleman of this town, of very consider- 

 able literary and scientific acquirements, and one of the Vice-Presi- 

 dents of our Natural History Society lately informed me that he 

 had been greatly surprised while walking past the iron railings of 

 our College, and at the same time looking through them at the snow 

 covered country, the sun also shining brightly, and being near the 

 horizon, to observe, that as the rails and open spaces came alternately 

 before his eyes, a very vivid play of colours presented itself to his 

 notice, the shades being chiefly reds and blues ; but he thought he 

 also observed other prismatic colours. On the next opportunity I 

 had, the snow, however, being only on the distant hills, I tried the 

 experiment myself, but as I walked at a slow pace, I was nearly sup- 

 posing that my eyes being very defective were not fitted to make the 

 observation, when I recollected that Mr. Patterson habitually walks 

 very fast ; I therefore quickened my pace very considerably, and was 

 much gratified at observing, after a few seconds; that very vivid 

 changes of colour did present themselves to my notice, but the pre- 

 dominant colour to me was a sjhade of pink, and some dark colours 

 which I could not accurately distinguish alternated with it. The 

 intervention of the opaque bars of the railing at equidistant intervals 

 was no doubt the cause, and the entire effect would seem to me to 

 prove that the retina takes different lengths of time to be impressed 

 with the several colours sufficiently for the notice of them to be con- 

 veyed to the mind, as also that the time during which the retina 

 remains under the influence of the impressions of the several coloured 

 lights after the light itself had ceased to act is different. If this be 

 the cause of the appearance I have attempted hastily to describe, it 

 can very easily be put to the test of accurate and direct experiment, 

 by an apparatus somewhat similar to the lately invented pleasing 

 illusion, the stroboscope." 



VII — Prevention of Dry Rot by Corrosive Sublimate. 

 Sir, 

 Having seen a notice of Dr. Faraday's idea of the innocence of Mr. 

 Kyan's application of the deutochloride of mercury, when used as a 

 preventive of the dry rot in timber, may I solicit your opinion re- 

 specting the following queries : 



1st. Will a ship built with timber, so prepared and sheathed with 

 copper, be deprived of such sheathing by the elective attraction be- 

 tween its atoms and those of the salt ? 



2nd. Will there not be a succession of decompositions and of re- 

 compositions exerted between the saline matter of the sea the copper- 

 sheathing and the salt, and of these sufficient to excite a galvanic 

 action by which such preparation will be rendered inert ? 

 Remaining, Sir, 



Your humble servant, 



J. H. L. 

 Rugby, April 8, 1835. 

 To the Editor of the Records of General Science. 



