MICROMETER EYE-PIECE. 123 



of the divisions of this scale must be determined, as in the for- 

 mer instance, by means of a ruled stage-micrometer, for each 

 objective employed in micrometry (the drawing out of the eye- 

 piece tube enabling the proportions to be adjusted to even and 

 convenient numbers) ; and this having been accomplished, the 

 scale is brought to bear upon the object to be measured, by moving 

 the latter as nearly as possible into the centre of the field, and 

 then rotating the eye-piece in such a manner, that the scale may 

 lie across that diameter which it is desired to measure. The push- 

 ing-screw at the extremity of the scale being then turned, until 

 one edge of the object is in exact contact with one of the long 

 lines, the number of divisions which its diameter occupies is at 

 once read off by directing the attention to the other edge, — the 

 operation, as Mr. Quekett justly remarks, being nothing more 

 than laying a rule across the body to be measured. This method 

 of measurement may be made quite exact enough for all ordi- 

 nary purposes, provided, in the first place, that the eye-piece 

 scale be divided with a fair degree of accuracy ; and secondly, 

 that the value of its divisions be ascertained (as in the case of 

 the cobweb-micrometer) by several comparisons with the scale 

 laid upon the stage. Thus if,- by a mean of numerous observa- 

 tions, we establish the value of each division of the eye-piece 

 scale to be l-12,500th of an inch, then, if the image of an object 

 be found to measure 3J of those divisions, its real diameter will 

 be 3J X T55TTI) or l-3571st of an inch.' ISTow as, with an objec- 

 tive of l-12th inch focus, the value of the divisions of the eye- 

 piece scale may be reduced to l-25,000th of an inch, and as the 

 eye can estimate a fourth 'part of one of the divisions with tole- 

 rable accuracy, it follows that a magnitude of as little as 

 l-100,000th of an inch can be measured with a near approach to 

 exactness, and that this instrument cannot be fairly considered as 

 ranking much below the cobweb-micrometer in minute accuracy. 

 At any rate, it is sufficiently precise (when due care is employed) 

 for all ordinaiy purposes; and it has the great advantage of 

 cheapness and simplicity. Whatever method be adopted, if the 

 measurement be made in the Eye-piece, and not on the stage, 

 it will be necessary to make allowance for the adjustment of the 

 object-glass to the thickness of the glass that covers the object, 

 since its magnifying power is considerably affected by the separa- 

 tion of the front pair of lenses from those behind it (§ 83). It 

 will be found convenient to compensate for this alteration, by 

 altering the draw- tube in such a manner as to neutralize the 

 effect produced by the adjustment of the objective ; thus giving 

 one uniform value to the divisions of the eye-piece scale, what- 



' The calculation of the dimensions is tnost simplified by the adoption of a decimal 

 scale ; the value of each division being made, by the use of the draw-tube adjustment, 

 to correspond to some aliquot part of a ten-thousandth or a hundred-thousandth of an 

 inch, and the dimensions of the object being then found by simple multiplication : — Thus 

 (to take the above example) the value of each division in the decimal scale is '00008, 

 and the diameter of the object is OOO^S. 



