2/6 ACCESSOEY APPARATUS 



If the answer is required in metrical measurement, then as 1 

 inch = 25*4 mm., 



(iv) 4-257 : 6'45 :: (-001 x25'4) : xmm. ; 



6-45 x '0254 -1638 



In this connection it will be as well to give two examples of 

 scale comparison which are sometimes required. Thus you have a 

 certain interval on a metrical stage micrometer which you know to 

 be accurate, and you wish to compare an English stage micrometer 

 with this scale in order to find out which particular interval of T^OTT 

 inch agrees with it. Suppose '05 mm. = 8'38 screw value as above, 

 then all that is necessary is to find the point to which the screw 

 micrometer must be set in order that it may accurately span the 

 y^Vo i ncn - Take 1 inch =25 -4 mm. as before ; then '001 inch= 

 0254. 



(v) -05mm. : '0254mm. :: 8'38 : x screw value; 



05 



Conversely, if a metrical scale is to be compared with an accurate 

 English one where '001 inch=4*257 screw value, then the screw 

 value for '05 mm. may be found thus : '001 iiich='0254 mm. 

 (vi) -0254mm. : '05mm. :: 4'257 : x screw value; 

 -05 x 4 ' 257 va i ue f or -05 mm. 



0254 



A cheap substitute for the screw micrometer has been devised by 

 Mr. G. Jackson. It consists in having a transparent arbitrary scale 



inserted into an or- 

 dinary Huyghenian 

 eye-piece in the focus 

 of the eye-lens, so 

 that it will be in the 

 same plane as the 

 magnified image of 

 the object to be 

 measured. It is seen 

 in fig. 215. The 

 method of using it is 

 precisely similar to 

 that of the screw 

 micrometer ; the 



value of T^Q-Q i ncn 01 ' 

 -jL mm., as the case 

 may be, is found in 



J 



FIG. 215. Jackson's eye-piece micrometer. terms of the arbitrary 



scale. The value of 



the object in terms of the same scale is also found, and comparison 

 made accordingly. All that need be done is to substitute the terms 

 of the arbitrary scale for screw values in the preceding examples, and 

 they will meet the case. 



