a new prifmatic Micrometer. 8 
moving the wedges with a fcrew. This was compleated 
about the middle of Auguft in the fame year, and then 
felt to the Royal Obfervatory. I have the honour to be, 
REVEREND SIR, 
Your obedient humble fervant, 
PETER DOLLOND. 
I HEREBY certify, that in the month of May, 
1776, the Rev. Mr. maskelyne, Aftronomer-royal, 
produced to me, at Mr. dollond’s houfe in St. Paul’s 
church-yard, and in his prefence, as a new invention of 
his own, an inftrument for meafuring fmall angles, con- 
lifting of two achromatic prifms or wedges applied be- 
tween the objedt-glafs and eye-glafs of an achromatic 
telefcope about 30 inches long, by moving of which 
wedges nearer to, or farther from, the objedt-glafs, the 
two images of. an object produced by them appeared to 
approach to, or recede from, each other, fo that the focal 
length of the obje<St-glafs became a fcale. for meafuring 
the angular diftance of the two images. 
London, 
Hoy. 27, 1777. 
ALEX*. AU.BE RT- 
