[ 419 1 
XXVII. fbs Defcription of Two new Micrometers. Ey 
Mr. Ramfden, Optician ; communicated by Jofeph 
Banks, Efq. P. R. S. 
HEN I offer the defcription of a new micrometer 
to the learned members of the Royal Society, I 
do not flatter my felf that mere novelty will entitle it to 
their patronage. Senfible how much the theory of aftro- 
nomy is limited by the imperfection of inftruments, I 
always incline to improve rather than invent, except 
when repeated examinations convince me, that the im- 
perfections arife from defeCt in principle as -well as in the 
conftruCtion. 
In confidering micrometers, when an obferver finds, - 
that with the micrometer, which depends on moveable 
parallel wires, he cannot meafure any diameter of a pla- 
net, except that which is at right angles to the direction 
of its apparent motion, he cannot withhold his pre- 
ference to that conftruCtion which meafures the angle 
by the feparation of the images. It appeared therefore j 
Read March 25, 1779. 
to > 
