139 



an inch, and the object which before measured nine will then measure 

 ten ; which, divided by 2000 gives the decimal fraction ,005. 



Enter in a table the length to which the tube is drawn out, and the 

 number of divisions on the eye-piece micrometer equivalent to an 

 inch on the stage ; and any measurements afterwards taken with that 

 micrometer and object-glass may, by a short process of mental arith- 

 metic, be reduced to the decimal parts of an inch, if not actually ob- 

 served in them. 



In ascertaining the value of the micrometer with a deep object- 

 glass the hundredth of an inch on the stage will occupy too much of 

 the field : the two-hundredth or five-hundredth should then be used, 

 and the number of divisions corresponding to that quantity be multi- 

 plied by two hundred or five hundred, as the case may be. 



The micrometer should not be fitted into too deep an eye-piece, for 

 it is essential to preserve clear definition. The middle eye-piece of 

 the London-made instruments is for most purposes the best, provided 

 the object-glasses be of the first quality; otherwise the eye-piece of 

 lowest power answers better. The lens above the micrometer should 

 not be of shorter focus than three-quarters of an inch, even with the 

 best object-glasses. 



Micrometers have generally been ruled in squares ; but when fine 

 they are with difficulty counted, and in no case possess an advantage 

 over the one above described ; for it is obvious that dimensions may 

 be taken in any direction by merely turning the eye-piece. 



It is almost needless to remark that this is not injured by the slits 

 cut in the tube ; consequently an additional one is not necessary ; and 

 the expense is confined to the slip of ruled glass and the fittings. 

 The means, therefore, of measuring minute objects with the utmost 

 accuracy, can be applied to any compound microscope for a few 

 shillings. 



In conclusion, I would observe that the fine focal adjustment made 

 by moving the object-glass alone, and thus altering the distance 

 between it and the eye-piece, would destroy the perfection of mi- 

 crometry were it otherwise attainable ; and the adjustment for thick- 

 ness of glass is a quantity by no means to be neglected. The value 

 of the micrometer should be found with the object-glass, set both for 

 uncovered and covered objects ;* and, during this operation the screw 



* The draw-tube should be adjusted so as to give the same value in both cases, 

 and the distance to which it is drawn out should be entered in the table : it will then 

 be easy to set it so as to give correct results in any state of the object-glass. 



