C 536 ] 
XLIX, Defer iption ~of a Method of mea^ 
furing Differences of Right Afeenfon and 
Declination^ with Dollond’s Micrometer^ 
together with other ?tew Applications of the 
fame : By the Rev, Nevil Mafkelyae, B, D, 
F. R, S, Afronomer RojaL 
Read Dec. 12, f | £ divided objed: glafs micromc- 
JL ter, as happily applied by the late 
Mr. John Dollond tq the objed end of a refleding 
telefcope, and now witli equal advantage adapted by 
the prefent Mr. Dollond, his fon, to the end of an 
achromatic telefcope, is fo eafy of ufe, and affords fo 
large a fcale, that it is generally looked upon by 
aftronomers as the mod convenient and exad inftru- 
ment for meafuring fmall diftances in the heavens. 
But, as the common wire micrometer is peculiarly 
adapted for meafuring differences of right afeenfion 
and-declination of celedial objeds, and is not near fo 
convenient or exad for meafuring their abfolute 
diftances j fo on the contrary the objed glafs micro- 
meter is peculiarly fitted for meafuring diftances, and 
has, I believe, generally been fuppoled incapable of or 
unfit for meafuring differences of right afeenfion and 
declination. Thus the two micrometers, as mutually 
fupplying each other’s defedts, have been efteemed 
both 
