An Improvement Proposed in the Cross Wires of Telescopes, by Dr. Wilson, of Glasgow. In aLetter to the Astronomer RoyalAuthor(s): Alexander WilsonSource: Philosophical Transactions (1683-1775), Vol. 64 (1774), pp. 105-107Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/105990 .

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XI. - r Improsemens p.ropofed ioe the crofx Fires of -7eleJ2opes, by Dr Wilfon, of GlaEgow. Ia a Letter to the- iqJXrono- er R*oyal.

S I R (>llege, Glafgow, Oflt t5s 72Se kedde,Dec. 23, Hage latelY put in praXtice a methi

1773 1 of inzproving the crofs wires, that are made ufe of in aftrononlical telelcopess which bas pleaSed me fo much, thet IlpropoSe, itl this letter, to give you an account of itt knowstlg that you trc alvvays warmly intereRed in the fuceeX af any thing of this llature.

It has been hitherto a defideratum to draw fil- uer wire fille enough for aIlronotnical uSes. .1 ced not mention the inconveniences, attending

our being ltmited in ehis particular, as they wills of their awn accord, occur to you. I therefore proceed to defcrii the mearls, 1 have fallen upon, of obviating the difficulty.- This, <sn pr-adrice, is extremely fitnple, and ccpnfilts in noehing but in {latteningthe fineR wires, which are now drawn.- I have made the experiment, upon fllarer wire, mthich ss maerked 5oo to the inch. Having prt

Yoo LXIV. P pared

f tos]

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[ I06 ] pared a fmall block of Reel, the face of w-hich

was made v-ery flat and fmooth, a number olF tht-

wsres were Ilretched acrofs it, at conf}derable inter- .S .

vaTs, by having their ends fa{lened, by pitchy ae

tach flde of the block. This done, I took another

blockfflof Reel? of the fame fize, the face of which*

had been made Iikewire Bats and the top of it

rounded, the better to deteine the Ilroke of the

hammer; upon a-pplying this, over the wires Sying

BpOIl the firfi block, vhich wtas £rmly fixed in a

wice, and giving a fmart ItroE w^th a hammer of

oua five pound we-ight, I found al;l of them flat-

texled in a rery eve-n manner.

That I might have no difficulty of fftting

thefe vrtress fao flattened, intoS the telefcope, I pur-

pofely made the face of thew flceelw blocles a fmall

matter narrower than the width of the braEs

ringe xns our tranfit-inRrument, uvpon which th-e

cro>Es wires are fixi. By this uleans the wires retained their- roundnefs at both ends, and fo were eaiily fixed acrofs the ring, by the fcrew-pins, when thBir fine edges regarded tlle eye. By means aaro of a fimple contrivance, which will readily tecur in praflvice, I made the horizontal-wire to go acrofs the otllerss fo as ju(t to touch them. This h-orizortal wlre was a round one, of soo to the i}ch, which I pu-rpofely u-fed along with the

others, that I might form fome judgment of the sffeEts of flattened- ones, wllen viewed along with it in the field. I accordingly found a very firiking diminetiotl of the vifible fubtenfe of theSe-twlres, when compared with the ronud one, aad tEiis fo

conEtderable

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[ 1Q7 ] cenfzderable, as I am perfuaded could not be obw tained with round wires, unlefis they could be drawn to two or three thoufand to the inch.

F.rom what I perceived, in the experiment;*of flattening the ^rires, I do not See but that th-i diminution, if it were requifite, might be catrred- a gr¢at deal further.

I am, &c.

ALEXANDER %'ILSON.

SII. tO

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