for registering images. Light enters through the cornea at the  front of the eyeball, travels through the lens, and falls on the  retina. The retina contains light sensitive cells that convert  light energy into electrical impulses that travel through nerves  to the brain. The brain interprets the electrical signals to form  images. There are two kinds of light-sensitive cells in the  eyes: rods and cones.  [Figure 9-6] The cones are responsible for all color vision, from  appreciating a glorious sunset to discerning the subtle shades  in a fine painting. Cones are present throughout the retina, but  are concentrated toward the center of the field of vision at the  back of the retina. There is a small pit called the fovea where  almost all the light sensing cells are cones. This is the area  where most “looking” occurs (the center of the visual field  where detail, color sensitivity, and resolution are highest). While the cones and their associated nerves are well suited  to detecting fine detail and color in high light levels, the rods  are better able to detect movement and provide vision in  dim light. The rods are unable to discern color but are very  sensitive at low light levels. However, a large amount of light  overwhelms the rods, and they take a long time to “reset”  and adapt to the dark again. There are so many cones in the  fovea that the very center of the visual field has virtually no  rods at all. Therefore, the middle of the visual field is not  very sensitive in low light. Farther from the fovea, the rods  are more numerous and provide the major portion of night  vision. Of all the senses, vision is the most important for safe flight.  Most of the things perceived while flying are visual or heavily  supplemented by vision. As remarkable and vital as it is,  vision is subject to some limitations, such as illusions and  blind spots. The more a pilot understands about the eyes and  how they function, the easier it is to use vision effectively  and compensate for potential problems. The eye functions much like a camera. Its structure includes  an aperture, a lens, a mechanism for focusing, and a surface  $_{Figure 9-6.}$  The human eye. $_{Figure 9-5.}$  The reduction of atmospheric pressure that accompanies  flying can produce physical problems for scuba divers. Vision in Flight 9-9 The  rods  and  cones  (film) of  the  retina  are  the receptors  which record  the image and  transmit it  through the  optic nerve  to  the brain for  interpretation. Rods and  Cones Fovea (All Cones) The  pupil  (aperture) is the opening at  the center of the  iris.  The size of the  pupil is adjusted to control the amount  of light entering the eye. PUPIL Light passes through the  cornea  (the  transparent window on the front of the  eye) and then through the  lens  to  focus on the retina. CORNEA Lens Iris Rod Concentration Retina Optic Nerve