C *79 ] 
they are drawn off from that fituation, the images 
will feparate more or lefs, according to the diftance 
they are drawn to. £y this means the images of 
two different objedts, or of different parts of the fame 
object, not very far from each other, may be brought 
to . a conta<5t or coincidence at the focus : and this 
coincidence may be view’d to a very great nicety with 
a proper eye-glafs. 
The meafure of the angle fubtended by the two 
objedts, whofe images are thus brought to a coinci- 
dence, depends upon three things: Firft, a careful ob- 
fervation of the coincidence of the images : Secondly, 
an exadt meafure of the diftance, which the glaffes are 
drawn out to from that fituation, which makes the image 
fingle: And, laftly, a true knowlege of the focal 
diftance of the glafs. How the angle is to be found 
from thefe meafures, and how it may like wife be 
come at, by viewing two land-objedts at a convenient 
diftance, will be fhewn hereafter in the explanation 
of the figure. It is eafy to underhand, in the mean 
time, that the angle will be meafur’d with more 
accuracy, in proportion to the length of the glafs, 
which is ufed for that purpofe ; but the difficulty of 
managing long telefcopes is no lefs apparent. There- 
fore the moft pradticable method of ufing this micro- 
meter to advantage, is, to apply the divided objedt-glafs 
to the objedt-end of a refkdting telefcope : for, as the 
apertures of thefe fort of telefcopes are large in pro- 
portion to their lengths, they will admit of very long 
glafles j nor will the meafures be any way affedted by 
the metals or glaffes, which the refledtor is com- 
pofed of : and the angles will be found in the fame 
manner, as though the images were view’d with a 
, Z 2 fingle 
