1897 THE MICROSCOPE. 147 
of the long arm of the sliding piece, 1-10 inch; the posi- 
tion of the screws at f, and g, is not altered; as the dis- 
tance between the knife and the short armis one-fifth 
the length of e, f, the knife is lowered 1-50 inch for a 
complete revolution. The micrometer plate has one 
hundred divisions hence each division represents a 
lowering of the knife of 1-5000 inch, or five micro-mill- 
imeters. By bending the long bar at the joint, n, the 
knife can be placed in position to cut specimens of a dia- 
meter of from 4inch to the largest that can be clamped 
in an ordinary specimen clamp. 
As the knife is so fixed in its socket that its edge is 
parallel to a line joining the points of the screws, fig. 1, 
f and g, and as these points rest on the plane of the glass, 
the edge of the knife must be parallel to the plane of the 
glass. This is true, whatever be the distance of the end 
fig. 1, e, of the long arm from the point of the microme- 
ter-screw. Furthermore as long as the micrometer-screw 
is in a given position in its thread, the distance of the 
knife from the plane of the glass is constant. Hence 
in successive motions of the sliding-piece across the glass 
plate, the edge of the knife describes successive planes 
each of which is parallel to the plane of the glass. The 
sections are therefore included between parallel planes; 
hence they are of even thickness and net wedge-shaped, 
as might be supposed. 
Of course with each alteration of the position of the 
knife, the degree by which itis tilted towards the plane 
of the glass, is altered; but the edge must remain par- 
allel to this plane because it remains parallel to the line 
of the screw-points at f and g. The knife is tilted to 
such a degree that in no position of the instrument does 
its lower surface become parallel to the upper surface of 
the specimen. 
For cutting frozen specimens, or specimens embedded 
in paraffin the edge of the knife must be placed perpen- 
