430 Mr. ramsden’s Defcription of 
fourth, or that which is neareft the eye. Let p be the 
diameter of the objeCt glafs, e the diameter of a pencil at 
m , and f the diameter of the pencil at the eye; it is evi- 
dent, that the axis of the pencils from every part of the 
image will crofs each other at the point m, and e, the 
width of the micrometer glafs, is to p the diameter of the 
object glafs as in a is to go, which is the proportion of 
the magnifying power at the point m, and the error 
eaufed by an imperfection in the micrometer glafs placed 
at rn will be to the error, had the micrometer been at o, 
as m is to p. 
Fig. 3d, reprefents the micrometer ; a a con vex or con- 
cave lens divided into two equal parts by a plane acrofs 
its center; one of thefe femi-lenfes is fixed in a frame b, 
and the other in the frame e, which two frames Aide on 
a plate K-, and are prefled againft it by thin plates act: the 
frames b and e are moved in contrary directions by turn- 
ing the button d ; l is a fcale of equal parts on the frame 
B ; it is numbered from each end towards the middle 
with 10, 20, &c. There are two verniers on the frame 
e, one at m, and the other at n, for the conveniency of 
meafuring the diameter of a planet,. &c. on both fides 
the zero. The firft divifion on both thefe verniers coin- 
cides at the fame time with the two zero’s on the fcale l, 
and. 
