876 '  M. C. Lea on some new Manipulations. 
The paper is then loosened, slipped a few inches along the tube 
and tightened again, keeping the lower edge exactly even. A 
very small difference in the diameter of the tube will cause the 
line to appear broken, as represented in fig. 3, instead of straight. 
A displacement from end to end amounting to one ;}> of an 
inch is easily observable, and as this difference corresponds to a 
unassisted eye, With an ordinary lens, a difference of ,},; of an 
in length which would bear this test when applied with the ut- 
most ri 
groove, touching its edges, a diamond peneil is drawn along the 
tube, the edge of the oat serving me rule, and a line is made 
from one end ef the space intended to be graduated to the other. 
The tube is then rotated on its axes a quarter of an inch, an 
another line is drawn parallel with the former. These lines serve 
