Here’s a fun experiment you can try with a friend. In a dark room, hold a candle about a foot away from your friend’s eye. Position the candle a few inches off to the side of the direction of the gaze, and have your friend look a little to one side, too.
Look at the variety of highlights. And remember: a highlight is a reflection of the light source on a wet or shiny surface.
Several highlights are visible. The brightest one (2) is a reflection of the candle on the outer surface of the cornea. The image is oriented right side up, a miniature image of the candle itself. Just under the main highlight at (2), you can see a little red area. That’s a reflection of the glowing red wax just below the candle flame.
The next brightest highlights, (1) and (4), are reflections off the eye fluid that pools up along the edges of the eyelid. Those highlights are directly across the pupil from each other.
Another faint highlight, (3), is visible to the right of (2). This highlight is a reflection off the back surface of the eye’s lens. If your subject changes the focus from near to far, that highlight will shift very slightly to the left and right as the shape of the lens changes. When the lens accommodates to different focal lengths, it’s mainly the back surface of the lens that changes shape.
There are two very faint intermediate highlights between (2) and (3), but they’re too dim to show up in this photo. One is a highlight off the back of the cornea, and the next would be off the front of the lens. You might see them if you try this yourself.
Photo by Jeanette (in candlelight @ 800 ASA)
More at Seeing the Light: Optics in Nature, Photography, Color Vision, and Holography, by David S. Falk, p. 149. (Thanks, Roberto)