finder

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One that finds: a finder of great hidden treasure.
2. n. A viewfinder.
3. n. A low-power, wide-angle telescope fixed to the body of a more powerful telescope and pointed in the same direction for initially locating an object to be observed.

Century Dictionary and Cyclopedia

1. n. One who or that which finds or discovers. Specifically— One who finds or determines after search or inquiry.
2. n. An inventor, deviser, or originator.
3. n. A poet.
4. n. In the customs, a searcher employed to discover goods imported or exported without paying custom.
5. n. A smaller telescope attached to a larger, for the purpose of finding an object more readily.
6. n. An extra lens or other device attached to a photographic camera for the purpose of showing on a small supplementary ground glass, or otherwise, the position of the picture in the field of the sensitized plate: used in cameras for making instantaneous pictures.
7. n. A microscopic slide divided by fine lines into a number of minute squares, used to locate exactly any point of especial interest in the field of the microscope. By noting the square which covers the point in question, the observer is enabled to bring it at once into view.

Wiktionary

1. n. One who finds, or discovers something.
2. n. An optical device, such as a viewfinder, used to locate a target or other object of interest

GNU Webster's 1913

1. n. (Astron.) One who, or that which, finds a small telescope of low power and large field of view, attached to a larger telescope, for the purpose of finding an object more readily, called also a finder telescope or finder scope.
2. n. (Micros.) A slide ruled in squares, so as to assist in locating particular points in the field of vision.

WordNet 3.0

1. n. optical device that helps a user to find the target of interest
2. n. someone who is the first to observe something
3. n. someone who comes upon something after searching