F'be Microfcope explained. li 
parts, the inch will by that means be fubdivided into 
two hundred equal parts. Laftly, if a b, c a, are made 
about three inches long, and divided into one hundred 
equal parts, we obtain a divifion of i-ioooth part of 
an inch, as in fig. 27. In this figure the breadth a b is 
i-ioth of an inch- the next divifion to it is 9,9-1 ooth 
parts of i-ioth; or in other words, it is i-ioooth part 
of an inch fhorter than i-ioth, and fo on till you come 
to the point c. 
The firft of thefe fcales was drawn on a filver bar, 
in form of fig. 25. 
This fcale has been fince changed by Mr. Cummins, 
thus ; inftead of making a folid bar in the {hape of fig. 
25. he made a perforation of the fame fhape, between 
the edges of two plates, as in fig. 27. which is more 
agreeable to the eye, when viewed in the microfcope, 
the light paffing thro’ the aperture ; and in the body of 
the microfcope to be ufed with this apparatus. At the 
field of view was placed a circle of i-6th of ail inch 
diameter, cut on a glafs plane, which is to be clipped by 
the inner edges of the fedtoral fcale, in order to come at 
the magnifying power of the inftrument, 
But this apparatus, as hitherto applied, is not fo ufeful 
as the following, therefore we Ihall proceed to give. 
The ufe of the micrometer and fedtoral fcale, 
when drawn on glafs or mufcovy talc. 
F IG. 28. reprefents the micrometer which we menti¬ 
oned before ; to place it upon the body of the mi¬ 
crofcope, open the circular part F K H, by taking out 
the fcrew g, and throwing back the femicircle F K 
which turns upon a joint at K, as may be feen in fig. 28. 
Now turn the upper tube of the body of the microfcope, 
fo that the fmall holes in both of them may coincide to 
■« 2 let 
