12 ON DIVIDING INSTRUMENTS. 



meters, a division of the head is of the value of about 0*2", 

 and the power and distinctnehs such, that when great care 

 is taken, a much greater errour than to the amount of one 

 of these divisions cannot well be committed in setting the 

 wire across the image of a well made dot. The double eye 

 glass has a motion by hand, for producing distinct vision 

 of the wire; and distinct vision of the dots is procured by 

 similar adjustment of the whole microscope. 

 Aftjastment of The first step towards sizing the roller is, to compute its 

 e Mze o 1 jjiajy,g(-gi. according to the measure of the circle, and to re- 

 duce it agreeably thereto, taking care to leave it a small 

 matter too large. The second step is, after having brought 

 the roller into its place in the plate B B, to make a mark 

 upon the surface of the circle near the edge, and a similar 

 one upon the roller, exactly opposite each other; then carry 

 the apparatus forward with a steady hand, until the roller 

 has made sixteen revolutions. If, now, the mark upon the 

 roller, by having overreached the one upon the circle, shows 

 it to be much too large, take it out of the frame, and re- 

 duce it by turning accordingly. When, by repeating this, 

 it is found to be very near, it may be turned about '001 of 

 an inch smaller on the lower edge, and so far its preparation 

 Ube & aflapta- is completed. The third and last step is, the use and 

 m"'^Q oues^° adaptation of the tv/o microscopes; one of these must take 

 its position at 11 in Fig. 1, viewing a small well defined dot 

 made for the purpose on the circle; the other, not repre- 

 sented in the figure, must also be fixed to the main plate 

 of Fig. 1, as n^ar to the former as possible, but viewing one 

 of the divisions on the roller. With a due attention to each 

 microscope, it will now be seen to the greatest exactness, 

 when, by raising or depressing the roller, its commensurate 

 diameter is found. 

 Apparatus for Fig. 3, PI. 11, is a representation of the apparatus for trans- 

 aiaking tha ferring the divisions of the roller to the circle. It consists of 

 jj_e.^ two slender bars, which, being seen edgewise in the figure, 



have only the appearance of narrow lines; but, when looked 

 at from above, ihey resemble the form of the letter A. 

 They are fastened to the main frame, as at W and Z, by 

 short pillars, having also the off leg of the angle secured 

 m the same manner; Y is a fine conical steel point for 



making 



