SCIENTIFIC NLWS. 319 



Spider's Webs to form the crofi Wires in the Eye Piece of AfirQ* 

 nomical and other Instruments* 



The fmalleft filver wire I have heard of was rather thinner Smalleft wirei 

 than one thoufandth part of an inch ; but every fpecimen I 

 have feen was confiderably thicker. This has not been fup- 

 pofed of much confequence in the construction of plumb lines ; 

 but it is very defirable that the wires in the focus of microf- 

 copes fliould be as fine as poffible. In a defcription of inftru- 

 ments in the Cabinet of the Grand Duke of Tufcany, arranged 

 fey the Abbe" Fontana, and written by Ami Angand in one of 

 the Journals de Phyfique above twenty years ago, to which I 

 mull refer the reader to the index of that ufeful work, not 

 having a whole fettat hand ; mention is made of fpider's webs Spider's webs 

 in the focus of microfcopes for meafuring ; and long tince, "opes? m ' Cr ° " 

 when upon obferving that the fingle thread of the filkwormis 

 very flender, I mentioned its ufe to my old friend J. H. de Silkworms 

 Magellan, F. R. S. he informed me, that organized fubftances threads * 

 had been tried in agronomical teleicopes, and rejected, be- 

 caufe the folar image was found to burn them. 



I luppofe this muft have been taken for granted by himfelf Troughton's ap- 

 or ibmebody elfe without trial-, for Mr. Edward Troushton, P ,i ^ tion 1 off P i - 



* ° der s webs to 



whole talents are indefatigably directed to every thing that aftronomical te- 

 can infure the preciiion and accuracy of aftronomical inftru- Scopes, 

 ments, has for fome time applied the webs of young fpiders 

 to the purpofe above-mentioned. He takes hold of a fhort 

 length of the web by means of a forked ftick, and then care- 

 fully applies it to the marks on the frame or ring intended to 

 hold it, where it adheres by the previous application of a lit- 

 tle varnifli. , Thefe lines have a beautiful afpect for their de- 

 licacy, preciiion, and evennefs. 



That the folar focus fhould not injure them is a fact of fome Curiousfaftthat 

 remark ; which I mould be difpofed to afcribe to their tranf- * he ******* le n* 

 parency, if I had not been aftiired by an eminent philofopher, fmall wire, 

 that fine metallic wires were not affected by Parker's great 

 lens, which inftantaneoufly fufed and burned thicker wires 

 and malfes. What may be the theory of this ? Will the afcend- Conjectures, 

 ing current of air cool a fmall wire more fpeedily than a larger, 

 of which the internal parts bear a greater proportion to the 

 fur face? or does the extreme light ignite, and perforate the 

 3 fmall 



