a new prifmatlc Micrometer. 803 
Let acb (fig. 2 .) reprefent the objeCt-glafs, and d the 
eve-glafs of a telefcope, and fr a prifm placed to in- 
tercept part of the rays coming from an object, fup- 
pofe the Sun, before they fall on the objeCt-glafs. The 
rays ee proceeding from the Eaftern limb of the Sun, 
and refracted through the objeCt-glafs acb without paf- 
fing through the prifm, will form the correfponding 
point of the Sun’s image at e; and the rays ww proceed- 
ing in like manner from the Wellern limb of the Sun 
will be refracted to form the correfpondent point of the 
Sun’s image at w. But the rays 2E, 2e, aw, zw, pro- 
ceeding in like manner from the Eaftern and Welfern 
limbs of the Sun, and falling on the prifm pr, and 
thence refracted to the objeCt-glafs acb, will, after re- 
fraction through it, form the correfpondent points of the 
Sun’s image at ze, zw. Let the refraction of the prifm 
be equal to the Sun’s apparent diameter : in this cafe, at 
whatever diftance the prifm be placed beyond the objeCt- 
glafs, the two images of the Sun we, zwze, will touch 
one another externally at the point ezw; for the rays 
2W, 2W, proceeding from the Weftern limb of the Sun 
being inclined to the rays ee proceeding from the 
Eaftern limb in the angle of the Sun’s apparent diame- 
ter, will, after fuffering a refraction in palling through 
the prifm equal to the Sun’s apparent diameter, emerge 
5 H 2 . from 
