An eye is logically prior to any optical instrument, and incommensurable with it. An instrument is something which is intended, in the last analysis, to be used by an eye. An optic array is the light converging to any position in the transparent medium of an illuminated environment insofar as it has different intensities in different directions. Differences in spectral composition may accompany the differences of intensity. An optic array is extended and enduring whereas the word “stimulus” suggests something punctate and momentary, like a pinprick. An optic array is the proper stimulus for an ocular system. In speaking of the optical stimulus one must distinguish between simultaneous action and successive action. The precise geometrical relation between the layout of opaque substances in space and the layout of luminous transitions in an optic array has to be determined by ecological optics. The use of the term “ambient” suggests the light that surrounds an individual on all sides.