[ 799 1 
XXXVI. Account of a new InJlrument for meafurin gfmall 
Angles , called the priftnatic Micrometer. By the Rev. 
Nevil Mafkelyne, D. D. F. R. 6V and AJlronomer - 
Royal. 
micrometer, for meafuring differences of right afcenfion 
and declination: nor did it receive lefs advantage from 
Mr. s avery’s moil ingenious invention of the divided 
obje<fl-glafs micrometer, which has been rendered more 
commodious by the late Mr. john dollond’s applica- 
tion of it to the obje£t-end of a reflecting telefcope, or 
the prefent Mr. peter dollond’s application of it to the 
objeCt-end of an achromatic refracting one. 
But, valuable as the objefr-glafs micrometer un- 
doubtedly is, fome difficulties have been found in the 
ufe of it, owing to the alterations In the focus of the eye, 
which are apt to caufe it to give different meafures of the 
fame angle at different times. For inftance, in meafuring 
the Sun’s diameter, the axes of the pencils of rays, which 
come through the two fegments of the object -glafs from 
Read Dec. 18, 
i m- 
R ACTICAL aftronomy was much be- 
nefited by the invention of the wire 
4 
contrary 
