The Governing Faculty

Ptolemy says that vision is not just colored light and images but also involves some precessing of what is received by the eye. The power of discernment involves a critical sense which he refers to as the Governing Faculty. Light is a visual flux and colors are related to objects but vision is capable of distinguishing different colors. When looking at a mirror one sees objects in the mirror. There are other optical illusions. The Governing Faculty tries to make sense what is seen [and sometimes fails to do so].

In assessing observations a scientist often needs modify direct measurements to refer them to an observer under standard conditions. The standard observer in astronomy is one in a vacuum at the center of the Earth. Observations are given relative to this reference point to make a comparison of them easier. In a way this is like the Governing Faculty. It forms a common basis for perception.

The closest that Ptolemy comes to discribing the action of a lens is in Bk III, 16 where he describes the shape of the surface of the eye as being like that of a convex mirror while talking about the illusory object in a mirror and its image. The eye or cornea is referred to as the viewer (aspiciens) (see the translation of Ptolemy's Optics by A. Mark Smith) but Ptolemy could just as easily be talking about the Governing Faculty. Ptolemy's model of the eye is that of geometrical optics with off axis rays corresponding to the points of an image. Elsewhere he uses this in reference to the breadth and height of an image and its distance. His model treats images as being three dimensional.