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Fig. 1. Diagram of an ideal refracting superposition eye in the dark-adapted state.
Light entering an eye first passes the corneal facet lenses (c) and
subsequently the crystalline cones (cc). Proximal to the array of crystalline
cones is the clear zone (cz) and the layer of rhabdoms (rh). A beam of light
parallel to the optical axis of an ommatidium is focused on the rhabdom of the
central ommatidium. Sheets of screening pigment surround the crystalline
cones. In many species, tracheolar tapeta and/or screening pigment isolate the
rhabdoms from each other. The set of facets that contributes to the
superposition image of a distant point source is called the superposition
aperture.