go On dividing Instrumevts. 



give a description of ihem. Fig. 6 is a section of the full 

 size, and snfficiciitlv explains their construction, and the 

 position of the glasses ; luit the micrometer part and man- 

 ner of mcimtine it, are hetter shown at H", in fig. 1 and 9. 

 The niiciometer part consists of an ohlong square frame, 

 which :s soldered into a slit, cut at right angles in the main 

 tube; another similar piece nicely fitted into the former, 

 and having a small motion at right angles to the axis of the 

 microscope, has at one end a cylindrical guide pin, and at 

 the other a micrometer screw j a spring of steel wire is also 

 applied, a'^ seen in the section, to prevent play, by keeping 

 the head of the nncronu tcr in close contact will) the .fixed 

 frame. This head is duided ir.to one l.nndred parts, which 

 are numbered each way to 50 ; the use of which will be 

 shown hereafter. A fine wire is stretched across the move- 

 able frame, for the purpose of bisecting fine dots. Two of 

 these microscopes are necessary; also a third, which need 

 not have the divided head, and must have in the moveable 

 frame two wires crossing each other at an angle of about 30°: 

 this microscope is shown at I, fig. 1. In the two first mi- 

 Ciomcteis, a division of the head is of the value of about 

 O "2, and the power and distinctness such, that when great 

 care is taken, a much greater error than to the apiount of 

 one of tliese divisions cannot well b? eommittcd in setting 

 the wire acruss the image of a well made dot. The double 

 eye-alass has a m.otion by hand, for producing distinct vision 

 of tire wire; and di-tinct vision of the dots is procured by a 

 similar adjustment of the who) , microscope. 



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

 diameter according to the measure of the circle, and to re- 

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

 pialter 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 llie 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, liy having over-reached the oneupon thecircle, shows 

 it i<) be inuch loo large, lake it out of the frame and reduce 

 it by lurnintr accorduioly : when, bv repeating lliis, it is 

 fomid to lie \ery mar, il may be turned about -001 of an 

 inch smaller on ti'e lower i.dge, and so tar its preparation is 

 completed. The ihiid and lii>i step is, the use and adapta- 

 tion of the two microscojies ; one of these nuisi take its po- 

 sition al H in fit;. 1, viev\ing a small well-defined dot made 

 for the purpose on the circle j the other, not represented in 

 8 the 



