This is a really interesting and comprehensive look at the physiological process that allows the human eye to see at night.  The article has been adapted by the  American Optometric Association from "Night Vision Manual for the Flight Surgeon", written by Robert E. Miller II, Col, USAF, (RET) and Thomas J. Tredici, Col, USAF, (RET).  The image is from Wikipedia.

Article is here: http://www.aoa.org/x5352.xml