STEREOSCOPIC PHOTOGRAPHY 



583 



As but a single reflection occurs, the resultant view is reversed from the right to 

 the left. Printing, for instance, reads backward. This can be remedied by ha^'ing the 

 prints reversed from the negatives. 



Polished metal or front-silvered mirrors should be used as a faint second image is 

 received off the front surface of ordinary mirrors. 



A refinement and extension of this type is used extensively for ^■iewing X-ray 

 stereographs. Transparencies are used and placed in the machine reversed. 



The most common form of mirror stereoscope is of the double-reflection type shown 

 in Fig. 7. It is extensively used in aerial photography work. The path of the light 

 is as shown in the figure. The double reflection eliminates the reversal effect of the 

 original Wheatstone stereoscope. 



It is possible to design this stereoscope for prints of any size. Prints of varying 

 size may be used in any given machine, provided focal lengths are approximately 

 correct. The use of large prints has the advantage of minimizing the obviousness of 

 the print texture with relation to the picture. 



/.eff view 



Righi- 



Fig. 



-Double-reflection type of mirror stereoscope which eliminates the reversal effect 

 of the Wheatstone type. 



The total distance that the light travels from the print to the eye corresponds to 

 the focal length of the instrument. As this is quite large in comparison to lens stereo- 

 scopes, considerable enlargment should be made of views taken with ordinarj'- cameras. 

 This can successfully be done as the probleni of overlapping does not present itself. 



For observing aerial photographs the machine is usually placed so that the observer 

 looks down on the prints. Such views usually have onlj^ a portion of their areas 

 in common. These are adjusted by eye until they fuse. 



A single large metal plate polished on both sides maj^ be used as a single mirror 

 stereoscope. The plate is placed between the eyes and normal to them. The eyes 

 look into the plate at the reflections of the prints. Here the prints are reversed from 

 right to left due to the single reflection. 



Many other types of mirror stereoscopes are possible. Unusual designs are occa- 

 sionally resorted to for some technical purpose. For general purposes the double- 

 reflecting type is undoubtedly the most satisfactory. 



Anaglyphs. — An anaglyph is a stereograph in which the two views are respectively 

 printed in complimentary colors. Red and green have been universally used for this 

 purpose. Since a picture of one pure color disappears when viewed through a glass of 

 the same color and is most visible, appearing black, through a complimentary color, 

 the views can be rendered mutually exclusive by a pair of spectacles of the same colors. 

 Thus, if the right view is red, the right spectacle must be green. The light spaces in 

 the scene appear in both views but, being complimentary colors, fuse as white. 



