1 *fhe Micro/cope explained t 
at right angles to the axis of the fcrew; its revolutions 
being counted on a fcale of one inch, divided into fifty 
parts. 
Thus every revolution of the micrometer fcrew, de¬ 
termines i-50th part of an inch, and the circumfe¬ 
rence of the circular plate, to which the index of this 
fcrew points, being divided into twenty equal parts, we 
have by infpedtion any certain number of thefe parts 
contained in any part of one fingle revolution of the 
micrometer fcrew : by this means we obtain one fingle 
thoufandth part of an inch. 
To the Hiding piece of this micrometer, is fixed a 
fmall needle in fuch a manner, that its point may traverfe 
acrofs the field of view, by which means we are enabled 
to meafure the length and breadth of the image of any 
objedt applied to the microfeope, but not that of the ob¬ 
ject itfelf, without another affiftance. 
This requires an attentive eye to watch the motion of 
the needle’s point, as it pafles over the image of a known 
part of an inch placed on the ftage; we acquire a very 
fmall part of an inch by help of the following fcale. Fig. 
25, 26, 27. in which the two lines c a, c b, with the 
fide a b, form an ifocelles triangle; c a, and e b, being 
each of them two inches long, and the fhort fide a b, 
exadfly i-iothof an inch. The two longfides ac, be, 
may be of any length we pleafe, and the fhort fide 
equal in length to 1 -ioth of an inch ; the longer fides 
may be faid to reprefent the lines of lines, upon a fee- 
tor opened to i-ioth of an inch : hence whatever num¬ 
ber of equal parts c a, c b, are divided into, their tranfi- 
verfe meafure will be fuch a part of i-ioth of an inch 
as their divifions exp'refs. Thus if it be divided into 
ten equal parts, this will divide the inch into one hundred 
equal parts, the firft divifion next c, will be — i-ioofh 
part of an inch, becaufe it is the tenth part of I-ioth 
of an inch. If thefe lines are divided into twenty equal 
parts, 
