422 Mr. ramsden’s Defcription of 
ror, any alteration in the eye of an obferver cannot 
affedt the angle meafured. 
It has, peculiar to itfelf, the advantages of an adjuft- 
ment to make the images coincide in a direction perpen- 
dicular to that of their motion ; and alfo of meafuring the 
diameter of a planet on both lides the zero, which will 
appear no inconfiderable advantage to obfervers who 
know how much eafier it is to afcertain the contadt of 
the external edges of two images than their perfedt coin- 
cidence. A fhort explanation of the annexed drawings 
will make the conftruction and the properties of this mi- 
crometer obvious. 
I divided the fmall fpeculum of a refledling telefcope, 
of Cassegrain’s conftrudtion, into two equal parts, by a 
plane acrofs its center ; and by inclining the halves of the 
fpeculum to each other on an axis at right angles to the 
plane that feparated them, I obtained two diftindt images. 
The fatisfadtion I received on the firft trial was checked 
by the apparent impoffibility of reducing this principle 
to pradtice. The angular feparation of the two images 
in this cafe being half the angular inclination of the two 
fpecula, it required an index of an unmanageable length, 
to allow the quantity of one fecond of a degree to be- 
come viiible. Some time afterwards, on reviling the 
principle, I conlidered, that if both the halves of the mir- 
4 
ror 
