Optical axes of eyes and other optical systems.

If a ray enters and leaves an optical system along the same straight line that line is an optical axis of the system. The number of optical axes that a system can have is none, one, or infinity. The purpose of the article is to show how to determine whether a system has an optical axis and to find the optical axis if it is unique and all the optical axes if there are an infinity of them. A simple system may have no optical axis or an infinity of them. A more complicated system is more likely to have a unique optical axis. The optical model is linear optics and the optical system may have refracting elements that are relatively decentered, separated, and astigmatic with non-aligned principal meridians. All the possible types of cases are treated in an appendix. In particular an example examines a simple eye that has an infinity of optical axes and a more realistic eye that has a unique optical axis.