The area where the optic nerve enters the eyeball has no rods  or cones, leaving a blind spot in the field of vision. Normally,  each eye compensates for the other’s blind spot.  Figure 9-7 provides a dramatic example of the eye’s blind spot. Cover  the right eye and hold this page at arm’s length. Focus the left  eye on the X in the right side of the windshield, and notice  what happens to the balloon while slowly bringing the page  closer to the eye. center can help compensate for this night blind spot. Along  with the loss of acuity (sharpness) and color at night, depth  perception and judgment of size may be lost. Balloon pilots, while not normally conducting flight  operations at night, can experience similar issues when flying  in low light conditions, particularly if there is haze or reduced  visibility. In those instances where the balloon is operated at  night, such as during a night “glow” or tether, night vision  can be immediately destroyed by the light from the burner.  It may take several minutes for the pilot to recover his or  her vision, time in which complete awareness of his or her  Another problem associated with flying at night or in reduced  visibility is empty-field myopia, or induced nearsightedness.  With nothing on which to focus, the eyes automatically focus  on a point just slightly ahead of the aircraft. Searching out  and focusing on distant light sources, no matter how dim,  helps prevent the onset of empty-field myopia.  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  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.  [Figure 9-8] When looking slightly off center, it becomes clearer and  brighter. Refer to  Figure 9-8.  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  Figure 9-8.  Night blind spot. $_{Figure 9-7.}$  The eye’s blind spot. Empty-Field Myopia Night Vision 9-10 Cones active Rods active Night blind spot