finder

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that finds.
• noun A viewfinder.
• noun 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.

from The Century Dictionary.

• noun One who or that which finds or discovers. Specifically— One who finds or determines after search or inquiry.
• noun An inventor, deviser, or originator.
• noun A poet.
• noun In the customs, a searcher employed to discover goods imported or exported without paying custom.
• noun A smaller telescope attached to a larger, for the purpose of finding an object more readily.
• noun 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.
• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Astron.) One who, or that which, finds
• noun (Micros.) A slide ruled in squares, so as to assist in locating particular points in the field of vision.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun optical device that helps a user to find the target of interest
• noun someone who is the first to observe something
• noun someone who comes upon something after searching