1) Essential thing 2) North
Found on
https://www.crosswordclues.com/clue/cardinal-point
[optics] In Gaussian optics, the cardinal points consist of three pairs of points located on the optical axis of a rotationally symmetric, focal, optical system. These are the focal points, the principal points, and the nodal points. For ideal systems, the basic imaging properties such as image size, location, and orientation are completely...
Found on
http://en.wikipedia.org/wiki/Cardinal_point_(optics)
No exact match found.
