So 2 Dr. maskelyne’s Account of 
meter led me to inquire, whether feme method might 
not he found of producing two diftinCt reprefentations of 
the Sun, or any other objeCt, which fhould have the axes 
of the pencils of rays, by which they are formed, diverg- 
ing from one and the fame point, or nearly fo : and it 
occurred to me, that this might be done by the refraction 
of a prifm placed to receive part of the rays proceeding 
from the objeCt, either before or after their refraction 
through the objeCt-glafs of a telefcope. If the prifm be 
placed without the objeCt-glafs, the rays that are re- 
fracted through it will make an angle with the rays 
that pafs belide it equal to the refraction of the prifm ; 
and this angle will not be altered by the refraction of the 
objeCt-glafs afterwards. Confequently, two images of an 
objeCt will be reprefented, and the prifm fo applied will 
enable us to meafure the apparent diameter of any ob- 
ject, or any other angular diltance which is equal to the 
refraCtion of the prifm. But if the prifm be placed 
within the objeCt-glafs, that is to fay, between the objeCt- 
glafs and eye-glafs, the angle meafured by the inftrument 
will vary according to the diltance of the prifm from the 
focus of the objeCt-glafs, bearing the fame ratio to the 
refraCtion of the prifm, as the diltance of the prifm from 
the focus bears to the focal length of the objeCt-glafs. 
Let 
