6o 



Microscopical Essays. 



to the axis of the fcrew. The revolutions of the fcrew are 

 counted on a fcale, which is an inch divided into fifty parts ; the 

 index to thefe divifions is, a flower de luce marked upon the 

 Aider, which carries the needle point aerofs the field of the micro- 

 fcope. Every revolution of the micrometer fcrew meafures & 

 part of an inch, which is again fubdivided by means of the 

 divifions on the circular plate, as this is divided into twenty equai 

 parts, over which the index paffes at every revolution of the 

 fcrew ; by which means, we obtain with eafe the meafure of one 

 thoufandth part of an inch : for 50, the number of threads on the 

 fcrew in one inch, being multiplied by 20 the divifions on the 

 circular plate, are equal to 1000 ; fo that each divifion on the cir- 

 cular plate mews, that the needle has either advanced or receded 

 one thoufandth part of an inch. 



To place this micrometer on the body of the microfcope, open* 

 the circular part F K H, Fig. 8, Plate II. A, by taking out the 

 fcrew G, throw back the femicircle F K which moves upon a 

 joint at K, then turn the Aiding tube of the body of the micro- 

 fcope, fo that the fmall holes which are in both tubes may exa&ly 

 coincide, and let the needle g of the micrometer have a free 

 pafiage through them ; after this, fcrew it faft upon the body by 

 the fcrew G. 



The needle will now traverfe the field of the microfcope, and: 

 meafure the length and breadth of the image of any object that is 

 applied to it. But further affiftance muft be had, in order to- 

 meafure the objecl itfelg which is a fubject of real importance ; 

 for though we have afcertained the power of the microfcope, and 

 know that it is fo many thoufand times, yet this will be of little 

 , affiftance 



