MICROMETER. 219 



follows that the divisions or lines, in the eye-piece micro- 

 meter, were joVt of an inch apart; the stage micrometer may 

 now be removed, and every object viewed with the micro- 

 meter eye-piece and the one-inch object-glass can be measured 

 to the y^ij^-g^ of an inch. Should, therefore, one of the ^^^^ of 

 the eye-piece micrometer be divided into four parts, then every 

 one of these divisions would be the -jTyVo- Again, if with an- 

 other power it was found that ^^^,^0 of an inch on the stage 

 micrometer coincided with ten of the spaces of the eye-piece 

 micrometer, then the value of each of the eye-piece micro- 

 meter divisions would be the 1-7^,^55 of an inch. It wUl be 

 found in practice, when high numbers are being observed, that 

 the stage movements are rather too coarse to bring the lines 

 in the two micrometers accurately over each other; the small 

 screw at the end of the brass, as shown in fig. 140, must then 

 be employed. We have hitherto spoken only of the numbers 

 in the eye-piece micrometer coming out even, but such is 

 rarely the case ; the chances are, that there will be a very 

 minute variation between any two or more of both sets of 

 lines. If it be not a matter of much moment to determine the 

 value of the divisons accurately, and if no two lines coincide, 

 the mean of a number of observations may be taken as an 

 approximation to the truth ; but by far the most desirable way 

 to remedy the evil is to employ the draw-tube, before described 

 at page 69 ; this, which must be graduated on one side, as 

 shown in fig. 42, into inches and tenths, may be pulled out so 

 as to make some of the lines in each micrometer agree. 



The value of the divisions in the eye-piece micrometer 

 should be found with all the object-glasses, and be put down 

 in a tabular form, as follows; and, if the instrument be pro- 

 vided with a draw-tube, the table should include the extent 

 to which it ought to be drawn out, in order to make the 

 value of the micrometer divisions even mmibers : — 



