(t new prifmatic Micrometer . 813 
% 
meter; and I take this opportunity of exhibiting to the 
Society the inftrument itlelf, mentioned in Mr. dol- 
lond’s letter as executed by himfelf according to my di- 
rections, and fent to the Royal Obfervatory in the month 
of Auguft 1776. 
Greenwich, 
December 1 1, 1777. 
TO THE REV. DR, MASKELYNE, 
REV. SIR, 
St. Paul’s Church-yard, 
Nov. 22, 1777. 
ACCORDING to your delire I fend the following 
particulars of the experiments which were made by your 
directions, for compleating a new kind of micrometer for 
meafuring fmall angles. About the beginning of April 
1776, 1 received your firlt directions refpeCting this mat- 
ter, which were to make two prifmatic glalfes or wedges 
of fuch angles that rays of light, which paifed through 
them, fhould be refraCted about 1 8' of a degree : thele 
were to be placed between the objeCt-glafs and eye-glafs 
of an achromatic telefcope about 30 inches long. The 
angular edges of the two prifmatic glalfes were to be 
placed in contact with each other; they were to be 
moved in a parallel pofition from the objeCt-glals to the 
focus 
