On dividing Instruments. 169 



The number of persons at all capable of dis'iding origi- 

 nally havL hitherto been very few ; the practice of it being 

 so liiuitud, ihat, in less than twice seven years, a man 

 could hardlv hope to become a vvorknian in this most diffi- 

 cult a;t. How tar I shall be considered as having sur- 

 mounu^d these ditHculties, I know not ; but if. by the ine- 

 thod here revealed, I have not rendered original dividing 

 aliiinn equallv easy with what copying was before, I have 

 speiu much labour, time, and ttiought in vain. I have no 

 doui)i indeed, that any careful workman who can divide irj 

 common, and has the ability to construct an astronomical 

 inslrum^^^nt, w'ill, by following the steps here marked out, 

 be a')le to divide it, the first time he tries, belter than the 

 most experienced workman, by any former method. 



If, instead of subdividing with the roller, the same thing 

 be performed with the screw, it will not give to dividino- by 

 the eye aiiv- very distinctive character: I have practised this 

 on arcs of circles with success, the edge being slightlv 

 racked, the scieu carrying forward an index with the re- 

 qu, site apparatus, and having a divided micrometer head; 

 the latter answers to the subdividing sector, and, being 

 used with a corresponding table of errors, forms the means 

 ot correcting the primitive points; but the roller furnishes 

 a more delicate action, and is by far more satisfactory and 

 expeditious. 



It is known to many that the six-feet circle, which I am 

 now at work upon for our Royal Observatorv, is to be di- 

 vided upon a broad edsie, or upon a surface at right angles 

 to I he usual | lane of division : the only alterations, which 

 will on tliat account be required, are, that ihe roller must 

 at! upm that plane which is usually divided upon ; which 

 ro'!c-r, being elevated or depressed, ma\ be adjusted to the 

 commensurate radius without beins: made conical, as was 

 necessary m thi. oiher case. The apparatus, similar to the 

 other, must heie be fixed immoveable to the irame which 



quantities are seen, that a seaman will sometimes cpmplain that two pair of ' 

 these lines will coincide at the same time ; and that may liap()en, and yet no 

 divibion of his instrument err, by more tlian —,',.„''' P^r' of an inc!i. All this 

 is applicable to judging of the coincidence ot lines witli eacli other, and 

 furnishes not the most lavuurablc display of-thc accuracy of vision. But with 

 the microscopes here described, where the wire bisects the image of a dot, 

 or a cross wire is made to intersect the image of a line, by an eve practised 

 in such matters, a coincidence may undoubtedly be ascertained to ^i,!iBBth 

 part of an incli. 1 am of opinion that as small a quantity may be rcndei ed 

 vliiible to the eye, as can by contact be made sensible to tlie touch ; but 

 whether Mr. Sineaton's y.„l^^ and my .^^-.^ be nt)t the same thing, 1 will not 

 determine ; the diderence between them, however, is what he would no more 

 lave in<.tcaiei to fcti, than I dare pretend ij jjtc. 



supports 



