J()5 ON DIYIDING ASTRONOMICAL INSTRUMENTS. 



Few persons The miniber of persons at all capable of dividing origi- 

 dtYUiirv'*ori"i ^-^^h' ^'^^'^ hitherto been very few; the practice of it being 

 Baliy. so limited, that, in less than twice seven years, a man could 



hardly hope to become a workman in this most difficult art. 

 How far I shall be considered as having surmounted these 

 difficulties, 1 know not; but if, by the method here re- 

 vealed, I have not rendered original dividing almost equally 

 easy with what copying was before, I "have spent much la- 

 bour, time, and thought, in vain. I have no douljt indeed, 

 that any careful workman who can divide in common, and 

 lias the ability to construct an astronomical instrument, 

 will, by following the steps here marked out, be able to 

 divide it, the first time he tries, better than the most expe- 

 rienced workman, by any former method. 

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



iastead'onhe' ^^ performed with the screw, it will not give to dividing by 

 wlkr. the eye any very distinctive chaiacter. I have practised 



this on arcs of circles with success, the edge being slightly 

 racked, the screw carrying forward an index with the requi- 

 site apparatus, and having a divided micrometer head ; the 

 latter answers to the subdividing sector, and, being used 

 ■with a corresponding table of errours, forms the means of 

 correcting the primitive points ; but the roller furnishes a 

 more delicate action, and is by far more satisfactory and 

 expeditious. 

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

 ^[ \,\^ f °?^ now at work upon for our Royal Observatory, is to be di- 

 dividing on its vided upon a broad edge, or upon a surface at right angles 

 ^^^' to the usual plane of division : The only alterations, which 



will on this account be required, are, that the roller must 

 act upon that plane which is usually divided upon ; which 



vourable display of the accuracy of vision. But with the microscopes 

 here deicribcd, where the wire bisects the image of a cot, or a cross wire 

 is made to iiilerser.t the image of a line, by an eye practised in such m.it- 

 ler>, a coincidence may undoubtedly be ascertained to ^.^.^^^ part of an 

 inch I am of opinidn, that as small a quantity may be rendered visible 

 to the eye, as cao by contact be made sensible to the touch ; but whether 

 Mr. Smcaton's ^-f^i-g-Ts- and my -j-^j-t^^ be not the same thing, I will not 

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

 more have preteiidtxl to feel, than 1 dare pretend to see, 



roller. 



