5 
tool. The reactive forces arising from continuous accel 
eration and change of direction of the moving mass of the 
system are bound to affect unfavorably the performance 
of the plotting instrument so equipped. 
Neither is there any illumination systems based on 
the reflective principle that could be called ideal. 
But among several known solutions there is one that comes 
fairly close to satisfy exacting demands regarding size, 
weight, stability, simplicity of design and optical per 
formance. This system 
is based on the amazing 
geometrical properties 
of the ellipse and the 
reflective properties 
of a concave ellipsoid, 
a body of rotation of 
elliptical cross section 
(Figure 2). 
The major axis of 
the basic ellipse is the 
polar axis of rotation. 
The minor axis of the ellipse generates the equatorial 
plane. The foci of the basic ellipse located on either 
side of the equatorial plane become the focal points of 
the ellipsoidal body. If its surface is made internally 
reflective the ellipsoid becomes a light condensing de 
vice which reflects any spacial ray that passes through 
one of the focal points toward the second focal point. 
A mirror surface of this property has a strong light 
gathering power since it produces an infinite number of 
images at the second focal point of a suitable light 
source placed at the first focal point. The size of