a new prifmatic Micrometer . 809 
From what has been fhewn it appears, that this in- 
ftrument, which may be properly called the prifmatic 
micrometer, will meafure any angle that does not exceed, 
the fum of the refractions of the prifms, excepting only 
very fmall angles, which cannot be taken with it on ac- 
count of the vanifhing of the pencils of rays at the junc- 
ture of the two prifms near the focus of the obje£t-glafs;. 
that it will afford a very large fcale, namely, the whole 
focal length of the object-glafs for the greateft angle 
meafured by it; and that it will never be out of adjuft- 
ment; as the point of the fcale where the meafurement 
begins (or the point of o) anfwers to the focus of the 
objedt-glafs, which is a fixed point for celeftial objects,, 
and a point very eafily found for terreftrial objects. All 
that will he neceffary to be done, in order to find the 
value of the fcale of this micrometer, will be to meafure 
accurately the diftance of the prifms from the focus 
when the inftrument is fet to meafure the apparent dia- 
meter of any object fubtending a known angle at the 
centre of the objeft-glafs, which may be eafily found by 
experiment, as by meafuring a bafe and the diameter of 
the object obferved placed at the end of it, in the man- 
ner pradticed with other micrometers : for the angle fub*- 
tended by this object will be to the angle fubtended by a 
celeftial objedt, or very remote land object, when the 
Vol. LXVII. 5 I diftance 
