The cones concentrated in the fovea centralis of the eye are  primarily responsible for vision in bright light.  [Figure 17-12 ]  Because of the high light level, rhodopsin, which is a  biological pigment of the retina that is responsible for both  the formation of the photoreceptor cells and the first events  in the perception of light, is bleached out causing the rod  cells to become less effective.  Mesopic vision is achieved by a combination of rods and  cones and is experienced at dawn, dusk, and during full  moonlight. Visual acuity steadily decreases as available light  decreases and color perception changes because the cones  become less effective. Mesopic viewing period is considered  the most dangerous period for viewing. As cone sensitivity  decreases, pilots should use off-center vision and proper  scanning techniques to detect objects during low-light levels.  Scotopic vision is experienced under low-light levels and  the cones become ineffective, resulting in poor resolution of  detail. Visual acuity decreases to 20/200 or less and enables  a person to see only objects the size of or larger than the  big “E” on visual acuity testing charts from 20 feet away.  In other words, a person must stand at 20 feet to see what  can normally be seen at 200 feet under daylight conditions.  When using scotopic vision, color perception is lost and a  night blind spot in the central field of view appears at low  light levels when the cone-cell sensitivity is lost.  The area where the optic nerve connects to the retina in the  back of each eye is known as the optic disk. There is a total  absence of cones and rods in this area, and consequently,  each eye is completely blind in this spot.  [Figure 17-14]  As a result, it is referred to as the blind spot that everyone  Figure 17-15  provides a dramatic example of the eye’s  blind spot.  has in each eye. Under normal binocular vision conditions  (both eyes are used together), this is not a problem because  an object cannot be in the blind spot of both eyes at the same  time. On the other hand, where the field of vision of one eye  is obstructed by an object (windshield divider or another  aircraft), a visual target could fall in the blind spot of the  other eye and remain undetected.  Figure 17-14.  Central blind spot.  Mesopic Vision  Scotopic Vision  Central Blind Spot  1. Hold this page at an arm’s length.  2. Completely cover your left eye (without closing or  pressing on it) using your hand or other flat object.  3. With your right eye, stare directly at the airplane on  the left side of the picture page. In your periphery, you  will notice the black X on the right side of the picture.  4. Slowly move the page closer to you while continuing  to stare at the airplane.  Figure 17-15.  The eye’s blind spot.  17-21  Pupil  Blind spot  Optic nerve  Retina  Center of vision  Right LEFT 