unsatisfactory for the point process, or they have to be operated at several 1000 
volts. However, this is accompanied by other operational drawbacks. The main 
  
    
  
Fig. 12. SEG I, 1934. Magnific- Fig. 13. SEG V, 1952. Magnification up 
ation up to 5 times. Minimum to 6.4 times. Minimum height 
height 2.8 m. Weight 430 kg. 2 m. Weight 320 kg. 80-watt 
| 120-watt lamp with mirror lamp with Fresnel lens con- 
condenser. denser. 
  
consideration is a good condenser. The large lens condensers used with 1000-watt 
lamps in the past (Fig. 11) have later been replaced by laminated concave mirrors 
of ellipsoid-like shape and large di- 
mensions (Fig. 12). Foreign instru- 
ments have frequently employed 1 or 
2 fans for the cooling of the film. 
For the first time in the design 
of photogrammetric instruments, the 
  
  
  
  
  
  
er. 
^n SEG V employs a pair of Fresnel- 
X lenses as a twin condenser. Fig. 9 
2 —— illustrates in what manner the 'com- 
0 Fig. 9. Formation of a Fresnel-type lens from posite lens’ or 'grooved lens’ may be 
a plano-convex lighting lens. supposed to have been designed by 
Bs cutting a plano-convex lens into 
ie annular sections and the subsequent appropriate composition of the rings. The 
E. two grooved lenses of the SEG are made of plastic, and the grooves are less than 
f | mm deep. We will not fail to mention that in view of the high demand to 
s uniform distribution of light, the manufacture of these grooved lenses with large 
Is diameters presents certain difficulties. The main advantages are low weight and 
  
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