186 MICROSCOPIC OBJECTS. 



ever, in estimating the value of the micrometer in the eye- 

 piece, it be found to be no equal number of divisions, then 

 the value must be found by the rule of three. Example : 

 If the division on the stage occupies i5|- divisions in the 

 eye-piece, then to find the diameter of the hair, say — 



as 15|- : '01 : : 5 : -00322. 

 the real diameter of the hair. 



For full particulars on this subject see ' Micrographia,' 

 chap. ii. and Appendix No. 1. 



Note. — After the value of the divisions in a micro- 

 meter eye-piece are determined, no change must be made 

 in the object-glasses or variations in the distances of any 

 of the lenses of the microscope : hence it follows that the 

 adjustment to focus must be by the rack and pinion, and 

 not by the fine-adjustment screw o. A table may be 

 readily constructed showing the value of the divisions 

 with each object-glass ; and thus by mere inspection the 

 real size of any object may be easily known. 



