SIGHT

   

   Sight is probably the most developed sense in humans, followed closely by hearing.

   

                                                Eye 

   

   

   The organ of vision is the eye. It has a complex structure consisting of a transparent lens
   that focuses light on the retina. The retina is covered with two basic types of
   light-sensitive cells-rods and cones. The cone cells are sensitive to color and are located in
   the part of the retina called the fovea, where the light is focused by the lens. The rod cells
   are not sensitive to color, but have greater sensitivity to light than the cone cells. These
   cells are located around the fovea and are responsible for peripheral vision and night vision.
   The eye is connected to the brain through the optic nerve. The point of this connection is
   called the "blind spot" because it is insensitive to light. Experiments have shown that the
   back of the brain maps the visual input from the eyes.

   

   The brain combines the input of our two eyes into a single three-dimensional image. In
   addition, even though the image on the retina is upside-down because of the focusing action of
   the lens, the brain compensates and provides the right-side-up perception. Experiments have
   been done with subjects fitted with prisms that invert the images. The subjects go through an
   initial period of great confusion, but subsequently they perceive the images as right side up.

   

   The range of perception of the eye is phenomenal. In the dark, a substance produced by the rod
   cells increases the sensitivity of the eye so that it is possible to detect very dim light. In
   strong light, the iris contracts reducing the size of the aperture that admits light into the
   eye and a protective obscure substance reduces the exposure of the light-sensitive cells. The
   spectrum of light to which the eye is sensitive varies from the red to the violet. Lower
   electromagnetic frequencies in the infrared are sensed as heat, but cannot be seen. Higher
   frequencies in the ultraviolet and beyond cannot be seen either, but can be sensed as tingling
   of the skin or eyes depending on the frequency. The human eye is not sensitive to the
   polarization of light, i.e., light that oscillates on a specific plane. Bees, on the other
   hand, are sensitive to polarized light, and have a visual range that extends into the
   ultraviolet. Some kinds of snakes have special infrared sensors that enable them to hunt in
   absolute darkness using only the heat emitted by their prey. Birds have a higher density of
   light-sensing cells than humans do in their retinas, and therefore, higher visual acuity.

   

   Color blindness or "Daltonism" is a common abnormality in human vision that makes it
   impossible to differentiate colors accurately. One type of color blindness results in the
   inability to distinguish red from green. This can be a real handicap for certain types of
   occupations. To a colorblind person, a person with normal color vision would appear to have
   extrasensory perception. However, we want to reserve the term "extrasensory perception" for
   perception that is beyond the range of the normal.