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Across 4. smell. 5. receptor cells best for seeing during the day. 7. a thin layer of tissue at the back of the eyeball. 9. the transparent gelatinous substance filling the eyeball behind the crystalline lens. 11. artists use this cues to create an impression of depth on a two-dimensional surface. 13. The small, circular, optically insensitive region in the retina where fibers of the optic nerve emerge from the eyeball. It has no rods or cones. 14. a circular whose diameter varies in response to the level of the light present. 15. a dense, white, fibrous membrane that, with the cornea, forms the external covering of the eyeball. 16. the minimun magnitud of stimulus that can be detected 50% of the time.

Down 1. a small region at the center of the retina. 2. each cone cell contains one type of photo pigment. 3. a receptor cells designed for seeing at night. 6. taste. 8. the difference in the views seen by each eye. 10. the process by which rods and cones convert environmental energy into neural messages. 12. the transparent outer covering at the front of the human eye.