248 MEASURING SCREW; &C. 



This was fitted The fcrew had a collar, which I could draw together fo as 

 cremeter plate, ^o hold it faft, and another collar, on which I put the di vided 

 and had a clip, plate, and after that an index firmly fixed on the fcrew. The 

 fcrew could be P^^*^^ ^'^'^ having liberty to turn on its collar, I could fallen it 

 held faft while independent of the fcrew, by having two thumb fcrews op- 

 fhiftedT ^" pofite each other, bearing on the periphery of the plate, fo 



that I could turn the fcrew only, or turn the plate only, or 



turn them all together. \ 



The fcrew was 'The operation was as follows: Having laid the frame I had 



turned, the to divide on the board, to which this apparatus was conneded, 

 quantity anfwer- ' ' 



ing to O.I inch; I brought the index to on the plate, and made it (the plate) 

 then clipped faft f^ft^ (hen turned the index two revolutions and ,66 of a revo- 

 meterfettozero: 'wtion: This gave me a divifion of ,J inch. I then faftened 

 the fcrew was the fcrew by bringing the collar to bind upon it; liberated the 

 moved as before- p'^te, and turned it until on the divifions came to the index, 

 then clipped, The plate was then faftened, and the (crew fet free and turned 

 mttirkT'^hl ^'^^ revolutions as before. The fcrew was then again faftened 

 fcrew then fet and the plate fet free, and brought to the index as before, 



free and moved, ^fj^^ ^.j^j^h the plate was faftened, and the fcrew fet free and 

 &c. &c, ' . . 



turned, &c. With a little addition of apparatus to make the 



faftenings and loofenings inftantaneous; this method may be 

 ufed to much advantage when a more elaborate or fcientific in- 

 ftrument cannot be obtained. 



I forgot in our converfation on the fubjecl of reflefling tele- 

 fcopes, to bring to your mind a falfe calculation of Mr. Ed- 

 wards's in his directions for making fpecula for reflefling tele- 

 fcopes, pubtifiied in the nautical almanack fome years fince. 

 Mr. Edwards Concerning the place of the eye ftop, his words are '' It is 



afferts that the abfolutelv necefl"arv for perfed vifion that the eye ftiould be 

 eye ftop of a re- . ^ . . 



flefting tele- applied to a fmali hole of a certain dimenfion, placed exactly 

 fcopeoughtto jj^ |(jg focus of the fingle eye-glafs, if the eye-piece confift of 



J)e in the focus ,. , ir-i , r r , ^ rr 



of theeyeglafs. one glals only, or elle in the compound focus ot the glades; 

 if it is conftru6ted wjth two, as is moft commonly praflifed;" 

 and afterwards fays, " Let the diftance of the eye-hole from 

 the eye-glafs, if it is a fingle one, be put as near as can be 

 attained by meafure, equal to the focal diftance of the eye- 

 glafs, &c." 

 But it ought not Surely Mr. Edwards muft have taken this on a bare fuppo- 

 SpSeklys;' ^^^'«"' without ever enquiring why. When we fpeak of the 

 but of rays from focus of a lens, it is generally underfiood to mean the focus 

 the vortex of the ^^ parallel rays, but the fopus of parallel rays does not deter- 

 ]nm" ^"'*' mine 



