You would pay for this if you were doing some work on the eye and perception. The purpose of the site is to provide information related to sight within a simple, engrossing, interactive environment. It covers the eye and its function, light and its impact on perception, views of reality and optical illusions. It is a unique resource suitable for work with pupils between the ages of 10 and 15.
http:library.thinkquest.orgC001464cgi-binview.cgi