e in
either diagram. (See GEOMETRY: _Descriptive_.)

In the stereoscope the two diagrams, by the combined use of which the
form of bodies in three dimensions is recognized, are projections of the
bodies taken from two points so near each other that, by viewing the two
diagrams simultaneously, one with each eye, we identify the
corresponding points intuitively. The method in which we simultaneously
contemplate two figures, and recognize a correspondence between certain
points in the one figure and certain points in the other, is one of the
most powerful and fertile methods hitherto known in science. Thus in
pure geometry the theories of similar, reciprocal and inverse figures
have led to many extensions of the science. It is sometimes spoken of as
the method or principle of Duality. GEOMETRY: _Projective_.)

DIAGRAMS IN MECHANICS.

The study of the motion of a material system is much assisted by the
use of a series of diagrams representing the configuration,
displacement and acceleration of the parts of the system.

_Diagram of Configuration._--In considering a material system it is
often convenient to suppose that we have a record of its position at
any given instant in the form of a diagram of configuration. The
position of any particle of the system is defined by drawing a
straight line or vector from the origin, or point of reference, to the
given particle. The position of the particle with respect to the
origin is determined by the magnitude and direction of this vector. If
in the diagram we draw from the origin (which need not be the same
point of space as the origin for the material system) a vector equal
and parallel to the vector which determines the position of the
particle, the end of this vector will indicate the position of the
particle in the diagram of configuration. If this is done for all the
particles we shall have a system of points in the diagram of
configuration, each of which corresponds to a particle of the material
system, and the relative positions of any pair of these points will be
the same as the relative positions of the material particles which
correspond to them.

We have hitherto spoken of two origins or points from which the
vectors are supposed to be drawn--one for the material system, the
other for the diagram. These points, however, and the vectors drawn
from them, may now be omitted, so that we have on the on