a new prifmatic Micrometer. 811 
faring angles not exceeding 36', and confequently fit for 
meafuring the diameters of the Sun and Moon, and the 
lucid parts and diftances of the cufps in their eclipfes * 
and another fet of prifms to meafure angles not much 
exceeding one minute, and confequently fit for meafur- 
ing the diameters of all the other planets. This latter 
fet of prifms will be the more convenient for meafuring 
fmall angles, on account of a fmall imperfection attend- 
ing the ufe of this micrometer, as before mentioned; 
namely, that angles cannot be meafured with it when the 
prifms approach very near the focus of the objeCt-glafs, 
the pencils of rays being there loft at the point where 
the prifms touch one another. 
Upon the principles that have been here explained, a 
prifm placed within the telefcope of an aftronomical in- 
ftrument, adjufted by a plumb-line or level, to receive all 
the rays that pafs through the objeCt-glafs, may conve- 
niently ferve the purpofe of a micrometer, and fuperfede 
the ufe both of the vernier fcale and the external micro- 
meter; and the inftrument may then be always fet to 
fome even divifion before the obfervation. Thus the 
ufe of a telefcopic level may be extended to meafure 
with great accuracy the horizontal refractions, the de- 
preflion of the horizon of the fea, and fmall altitudes 
and depreffions of land objects. Time and experience 
5 I 2 will 
