ON DIVIDING INSTRUMENTS. 15 



making' the clots, and X is a feeler, whereby the point Y 

 n)ay be pressed down with a uniform force, which force 

 may be adjusted, by bending the end of the bar just above 

 the point, so as to make the dots of the proper size. The 

 point Y yields most readily to a perpendicular action ; but 

 is amply secured against any eccentric or lateral devia- 

 tion. 



The apparatus, so far described, is complete for laying Primary dots, 

 our foundation, i.e. making 256 primary dots; no matter 

 whether with perfect truth, or not, as was said respecting 

 the divisions of the roller; precision in either is not to be 

 expected, or wished; but it is of some importance, that 

 they should be. all of the same size, concentric, small, and 

 round. They should occupy a position very near the ex- 

 treme border of the circle, as well to give them the greatest 

 radius possible, as that there should be room for the sta- 

 tionary microscope and other mechanism, which will be de- 

 cribed hereafter. 



It must be noticed, that there is a clamp and adjusting 

 screw attached to the main plate of Fig. 1 ; but, as it differs 

 in no respect from the usual contriv;mces for quick and slow 

 motion, it has been judged unnecessary to incumber the 

 drawing with it. 



Now, the roller having been adjusted, with one micro- Method of 

 scope H upon its proper dot on the circle, and the other "taking thesi- 

 microscope at the first divii-ion on the roller; place the ap- 

 paratus of Fig. 3 so that the dotting point Y may stand di- 

 rectly over the place which is desigaed for the beginning of 

 the divisions. In this position of things, let the feeler X be 

 pressed down, until its lower end comes into contact with 

 the circle ; this will carry down the point, and make the 

 first impression, or primary dot, upon the circle; unclamp 

 the apparatus, and carry it forwards by hand, until another 

 division of the roller comes near the wire of the microscope; 

 then clamp it, and with the screw motion make the coinci-» 

 dence cosnplete; wheie again press upon the feeler for the 

 second dot: proceed in this maimer until the whole round 

 is completed. 



From these 256 erroneous divisions, by a certain course of Erroursoftliese 

 examination, and by computation, to ascertain their absolute "^^^^^ 5" ^^ ^^ 



and 



