The iris is the colored part of the eye that surrounds the pupil. 

 The iris changes in size as a function of the amount of light.  Bright light makes the pupil get smaller (the iris gets larger) and vice versa.  The pupillary area change from minimum to maximum is about 16:1.  Therefore this factor only accounts for a small portion of the ability of the visual system to adapt to light or darkness.  See the topic of dark adaptation for additional explanations.

One other interesting thing that the iris does is to vary the depth of field, in much the same was as a camera aperture does.  The smaller the pupil (the larger the iris) the greater the depth of field.

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