deposition of an aluminum coating becomes the reflecting 
condensing system of 120° aperture specified above. 
The Mapping Projector 
As an integrated member of 
the Balplex family of mapping 
projectors the new 120° addi 
tion (Figure 4) makes use of 
the same size diapositive 
2 
images on 110 x 110mm glass 
plates, the same printer and 
reduction ratio. The resulting 
principal distance of this pro 
jector is 31.80mm. Glass 
plates of J" thickness are 
preferred for their higher 
stability and surface flatness, 
ventional plateholders, centered therein on a standard 
centering device and, when inserted in the projector, 
occupy an unambigious position by which the correct re 
lation to the projection lens (interior orientation) 
is rigorously established. The projection lens is 
mounted in a ball socket. Its center coincides with the 
rear nodal point. It can be universally canted from 
its initial position. It produces imagery of critical 
focus at a nominal projection distance of 400mm in a 
plane which is either normal upon or oblique to the axis 
of projection, depending upon the controlled magnitude 
of lens cant introduced to establish a depth zone 
commensurate with the degree of obliquity of the aerial 
photograph. The accepted depth zone is -18% of the 
nominal projection distance or -70mm. 
Figure 4 
They are held in con-