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The Blind Spot

All the nerve impulses generated in the retina travel back to the brain by way of the axons (nerve fibres, which carry messages) in the optic nerve. At the point on the retina where the approximately 1 million axons converge on the optic nerve, there are no rods or cones. This spot, called the blind spot, is therefore insensitive to light.

You can exhibit the presence of the blind spot. Cover your right eye with your hand and stare at the red circle as you move closer to the screen (the square will disappear). Or cover your left eye and stare at the red square as you move.

Four kinds of light-sensitive receptors are found in the retina:

• rods
• three kinds of cones, each tuned to absorb light from a section of the spectrum of visible light
  • cones that absorb long-wavelength light (red)
  • cones that absorb middle-wavelength light (green)
  • cones that absorb short-wavelength light (blue)