expose the rods to the image. This can be done by looking 5°  to 10° off center of the object to be seen. This can be tried in  a dim light in a darkened room. When looking directly at the  light  it dims or disappears altogether. When looking slightly  off center  it becomes clearer and brighter.  When looking directly at an object  the image is focused  mainly on the fovea  where detail is best seen. At night  the  ability to see an object in the center of the visual field is  reduced as the cones lose much of their sensitivity and the  rods become more sensitive. Looking off center can help  compensate for this night blind spot. Along with the loss of  Another way to check your blind spot is to do a similar test  outside at night when there is a full moon. Cover your left  eye  looking at the full moon with your right eye. Gradually  move your right eye to the left (and maybe slightly up or  down). Before long  all you will be able to see is the large  halo around the full moon; the entire moon itself will seem  to have disappeared.  Empty-field myopia is a condition that usually occurs when  flying above the clouds or in a haze layer that provides  nothing specific to focus on outside the aircraft. This causes  the eyes to relax and seek a comfortable focal distance that  may range from 10 to 30 feet. For the pilot  this means  looking without seeing  which is dangerous. Searching out  and focusing on distant light sources  no matter how dim   helps prevent the onset of empty-field myopia.  There are many good reasons to fly at night  but pilots must  keep in mind that the risks of night flying are different than  during the day and often times higher.  [Figure 17-16]  Pilots  who are cautious and educated on night-flying techniques  can mitigate those risks and become very comfortable and  proficient in the task.  It is estimated that once fully adapted to darkness  the rods are  10 000 times more sensitive to light than the cones  making  them the primary receptors for night vision. Since the cones  are concentrated near the fovea  the rods are also responsible  for much of the peripheral vision. The concentration of cones  in the fovea can make a night blind spot in the center of the  field of vision. To see an object clearly at night  the pilot must  17-22  Night Vision  Night Blind Spot  Empty-Field Myopia  5. When the page is about 16–18 inches from you  the  black X should disappear completely because it has  been imaged onto the blind spot of your right eye.  (Resist the temptation to move your right eye while  the black X is gone or else it reappears. Keep staring  at the airplane.)  6. As you continue to look at the airplane  keep moving  the page closer to you a few more inches  and the black  X will come back into view.  7. There is an interval where you are able to move the  page a few inches backward and forward  and the black  X will be gone. This demonstrates to you the extent  of your blind spot.  8. You can try the same thing again  except this time with  your right eye covered stare at the black X with your  left eye. Move the page in closer and the airplane will  disappear.  Figure 17-16.  Night vision.  Figure 17-17.  Night blind spot.  Cones active  Rods active  Night blind spot  Pilots must look 5°–10° off center of the  object in order for the object to be seen. 