on Dividing astronomical instruments. 4g 



it could riot be brought hear enough to the axis of the mi- Method of as- 

 troscope ; and then part of the rays, which form the image ^onby^onti- 

 of the object seen by the microscope, will be intercepted by nued bisection. 

 the point; but I believe, that by proper management this 

 may be done without either making the point too weak, or 

 making the image indistinct; but if this cannot be done, we 

 may have recourse to Mr. Troughton's expedient of bisect- 

 ing an odd number of contiguous divisions. 



It must be observed too, that, in the bisections of all the 

 arches of the same magnitude, the position of the point d 

 on the frame remains unaltered; but its position must be 

 altered every time the magnitude of the arch is alteved. 



It is scarcely necessary to say, that the bisections thus 

 made are not intended as the real divisions, but only as marks 

 from which they are to be cut. In order to make the real 

 divisions* the microscope must be placed near the point, 

 and the support d must be placed so that d § shall be a tan- 

 gent to the circle at 5. The wire of the microscope must 

 then be made to bisect one of these marks, and a point or 

 division cut with the point, and the process continued trll 

 the divisions are all made. 



It is plain that in this way, without some farther precau- 

 tion, we must depend on the microscope not altering its 

 position in respect of the point during the operation; for 

 which reason I should prefer placing the axis of the micro- 

 scope at exactly the same distance from the centre of motion 

 d, as the point ; but removed from it sideways, by nearly the 

 semidiameter of the object glass; so that having made the 

 division, we may move the beam compass till the division 

 comes within the field of the microscope, and then see whe- 

 ther it is bisected by the wire, and consequently see whether 

 the microscope has altered its place. 



In the operation of bisection, as above described, it may 

 be observed, that, if the two scratches are placed so near 

 together, that in making the second the point of the C6m- 

 pass riins into the burr raised by the first, there seems to be 

 some danger, that the point may be a little deflected from 

 its true course; though in Bird's account of his method, I 

 do not find that he apprehends any incortvenh nee from it. 

 One way of obviating this inconvenience, if it does exist, , i 



Vol. XXVI.— -May, 1810. E ' would 



