!)('» 



APPENDAGES TO THE MICROSCOPE. 



object appears to be 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 



Jackson's Eye-piece Micrometer. 



may be made quite exact enough for all ordinary purposes, pro- 

 vided, in the first place, that the Eye-piece Scale be divided with a 

 fair degree of accuracy ; and secondly, that the value of its divi- 

 sions 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 observations, we establish the value of 

 each division of the eye-piece scale to be 1-12, 500th of an inch, 

 then, if the image of an object be found to measure 3g of those 

 divisions, its real diameter will be 3| X T ^ or l-3571st of an 

 inch.* With an Objective of l-12th-inch focus, the value of the 

 divisions of the Eye-piece Scale may be reduced to 1-25, 000th of 

 an inch ; and as the Eye can estimate a fourth part of one of the 

 divisions with tolerable accuracy, it follows that a magnitude of as 



* The calculation of the dimensions is much 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 dimen- 

 sions 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 "00028. 



