CAMERAS 



87 



An orange-red filter may be placed over one of the range-finder ports so that a deep 

 color is imparted to one of the images, increasing the ease of accurate focusing by an 

 increase in contrast between the two images. 



The base of the Leica range finder is approximately 40 mm. 



Zeiss Coupled Range Finder. — The principle of this type of finder is shown in 

 Fig. 19. 



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Fig. 18. — Leica range finder as fitted to camera. 



"At P' there are two prisms of triangular cross section and very small angle. 

 Viewed in the direction of the arrow these prisms would look like the inset in Fig. 19. 

 In the figure these prisms are shown in such a position that a ray of light such as OP 

 will pass through them without any change in direction, and the ray would go along 

 the line OE into the eye at E; together they act 

 here like a sheet of glass with parallel faces. These 

 prisms are arranged so that they will rotate 

 about the line OPE in opposite directions and at 

 equal rates as the focusing device of the camera is 

 adjusted. Having been turned for a little way, 

 the pair no longer act like a sheet of plane parallel 

 glass, but like a single prism, since now one side 

 of the combination is thicker than the other. 

 This can easily be demonstrated by using the 

 pointed ends of two lead pencils to represent the 

 prisms, holding the pencils perpendicular to 

 the plane of Fig. 19, and rotating each towards the 

 right-hand side of the figure, in opposite directions. 

 The result is that the ray of light is caused to 

 deviate from the path OPE to the path OPE'. In 

 this case mirror G is fixed, as is the sheet of glass 

 C, so that the ray OGC passes into the ej^e at E'. 

 It is obvious that by rotating the prisms as 

 described the two images can be brought into 

 coincidence along the line PE'." 



In the Zeiss range finder, use is made of the 

 fact that the color of light passing through a very jpjq^ jg _ 

 thin metallic layer is complementary to the color 

 of the light refiected by the layer. The colors into which white light are split by a 

 semitransparent gold coating are red and green which, when mixed, become white 

 again. 



The image-dividing mirror is given a coating of gold so that the objects in the 

 image will be seen in nearly their natural colors in the central field when the camera is 

 properly focused because the green and the red portions of the image will be rejoined 

 then. The mechanical details of the range finder are shown in Fig. 20. 



EE' 

 -Zeiss coupled range finder. 



