a focusing aid, found in manual focus cameras: it consists of a pair of prism wedges on the focusing screen. The wedges split an out-of-focus image into two halves laterally displaced relative to one another. The digree of displacement is proportional, of course, to how out of focus the image is.
When the lens is correctly focused the image becomes continuous.
The split-image wedge (also known as the split image telemeter) requires a contrasty subject, if possible with some definite line. It can help with focusing images where the ground glass cannot be used.

An example: what you see in the viewfinder, when focusing on a flower, rendered in garish ASCII art.

The subject:

  * *
 * * *
  * *
   |/
  \|  
   |

Through the viewfinder, unfocused (the silly horizontal line is the split-image wedge):

     * *
    * * *
_____* *_
   |/
  \|
   |

Through the viewfinder, now in focus:

  * *
 * * *
__* *____
   |/
  \|
   |

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