enlarge to receive as much of the available light as possible.  After approximately 5 to 10 minutes, the cones become  adjusted to the dim light and the eyes become approximately  100 times more sensitive to the light than they were before the  dark room was entered. Much more time, about 30 minutes, is  needed for the rods to become adjusted to darkness, but when  they do adjust, they are about 100,000 times more sensitive to  light than they were in the lighted area. After the adaptation  process is complete, much more can be seen, especially if  scanning techniques are used correctly. After the eyes have adapted to the dark, the entire process  is reversed when entering a lighted room. The eyes are first  dazzled by the brightness, but become completely adjusted  in a very few seconds, thereby losing their adaptation to the  dark. Now, if the dark room is re-entered, the eyes again go  through the long process of adapting to the darkness. In addition to night vision limitations, night illusions can  cause confusion and distractions during night flying. The  following discussion covers some of the common situations  that cause illusions associated with night flying.  Before and during night flight, the adaptation process of  the eyes must be considered. First, adapt to the low level of  light and then stay adapted. After the eyes are adapted to the  darkness, avoid exposing them for more than one second to  any bright white light as that causes temporary blindness. If exposed to a bright light source, such as search lights and  landing lights, remember that each eye adapts to the dark  independently. By closing or covering one eye when exposed  to light, some night vision acuity is retained in the closed eye. On a clear night, distant stationary lights can be mistaken  for stars or other aircraft. Cloud layers or even the northern  lights can confuse a pilot and indicate a false visual horizon.  Certain geometrical patterns of ground lights, such as a  freeway, runway, approach, or even lights on a moving  train, can cause confusion. Dark nights tend to eliminate  reference to a visual horizon. As a result, pilots need to rely  less on outside references at night and more on flight and  navigation instruments. Temporary blindness, caused by an unusually bright light,  may result in illusions or after images until the eyes recover  from the brightness. The brain creates these illusions  reported by the eyes. This results in misjudging or incorrectly  identifying objects, such as mistaking slanted clouds for the  horizon or populated areas for a landing field. Vertigo is  experienced as a feeling of dizziness and imbalance that can  create or increase illusions. The illusions seem very real and  pilots at every level of experience and skill can be affected.  Recognizing that the brain and eyes can play tricks in this  manner is the best protection for flying at night. Visual autokinesis can occur when staring at a single light  source for several seconds on a dark night. The result is that  the light appears to be moving. The autokinesis effect will  not occur if the visual field is expanded through scanning  techniques. A good scanning procedure reduces the  probability of vision becoming fixed on one source of light. Good eyesight depends upon physical condition. Fatigue,  colds, vitamin deficiency, alcohol, stimulants, smoking, or  medication can seriously impair vision. Keep these facts in  mind and take adequate precautions to safeguard night vision.  In addition to the principles previously discussed, the following  items aid in increasing night vision effectiveness. Distractions and problems can result from a flickering light  in the flightdeck, anti-collision light, or other aircraft lights  and can cause flicker vertigo. If continuous, the possible  physical reactions can be nausea, dizziness, grogginess,  unconsciousness, headaches, or confusion. Try to eliminate  any light source causing blinking or flickering problems in  the flightdeck. A black-hole approach occurs when the landing is made  from over water or non-lighted terrain where the runway  lights are the only source of light. Without peripheral visual  cues to help, orientation is difficult. The runway can seem  out of position (down-sloping or up-sloping) and in the  worst case, results in landing short of the runway. If an  Night Illusions 10-3 • Close one eye when exposed to bright light to help  avoid the blinding effect. • Do not wear sunglasses after sunset as this impairs  night vision. • Move the eyes more slowly than in daylight. • Blink the eyes if they become blurred. • Concentrate on seeing objects. • Force the eyes to view off center using scanning  techniques. • Maintain good physical condition. • Avoid smoking, drinking, and using drugs that may  be harmful. • Adapt the eyes to darkness prior to flight and keep  them adapted. About 30 minutes is needed to adjust  the eyes to maximum efficiency after exposure to a  bright light. • If oxygen is available, use it during night flying. Keep  in mind that a significant deterioration in night vision  can occur at cabin altitudes as low as 5,000 feet.