eliminate, the registry of scanping "tracks" on the negative. These fine 
lines, caused by voids (nonexposure) and overlaps (double exposures), F 
often appeared on the orthophotographs prepared with the earlier instruments, e 
and were evidence that, due to mechanical limitations, the exposing aper- f 
ture failed to traverse exactly contiguous areas of the negative as ad- i 
jacent strips were scanned. Although these lines do not disturb the Y 
position of images, their presence degrades the appearance of the product. c 
As shown in figure 2,the new instrument structure is designed 
to accommodate the supporting bar both for Kelsh-type stereoplotting " 
projectors and for the ellipsoidal-reflector type. Because the Geological T 
Survey has as abundance of both types in constant use, the diapositives i 
available for a given area are likely to be compatible with one but not i 
the other of these types of projectors. For this reason, the flexibility a 
in the type of projector that can be used with the Orthophotoscope con- 
stitutes a decided operational advantage. F 
Experience has indicated that the efficiency and the product s 
of scanning is enhanced when the direction of scanning conforms with the : 
‘ 0 
«) S 
i 
a 
1 
Y 
e 
a 
1 
t 
c 
a 
C 
C 
t 
t 
a 
C 
C 
t 
f 
t 
G 
C 
C 
f 
f 
V 
C 
K 
f 
      
1960 Orthophotoscope with Kelsh-type projeccors. 
4