Two new Micrometers. 423 
ror turned on their center of curvature, there could be 
no alteration in their relative inclination to each other 
from their motion on this center; and that any extent of 
fcale might be obtained, by fixing the center of motion 
at a proportional diftance from the common center of 
curvature. This will be better underftood from the an- 
nexed drawing. 
a (fig. 1.) reprefents the fmall fpeculum divided into 
two equal parts ; one of which is fixed on the end of the 
arm b ; the other end of the arm is fixed on a fteel axis x, 
which crofles the end of the telefcope c. The other half 
of the mirror a is fixed on the arm d, which arm at the 
other end terminates in a focketjy, that turns on the axis, 
x; both arms are prevented bending by the braces a a. 
g reprefents a double fcrew, having one part e cut into 
double the number of threads in an inch to that of the 
part gs the part e having 100 threads in one inch, and 
the part g 50 only. The fcrew e works in a nut F in the 
fide of the telefcope, while the part g turns in a nut H, 
which is attached to the arm b; the ends of the arms 
E and d, to which the mirrors are fixed, are feparated 
from each other by the point of the double fcrew pref- 
fing againft the ftud b, fixed to the arm d, and turning in 
the nut h on the arm b. The two arms b and d are 
preffed againft the direction of the double fcrew eg by 
K k k 2 a fpiral 
