In 1970, Gordon Gallup Jr developed the mirror test, which measures self-awareness by determining if an animal is able to recognize its own reflection, and interpret it as an image of itself. Prior to taking this test, the animal is marked with two dye spots, one of which can only be seen in the mirror, whereas the other one can also be seen directly. If the animal seems to be aware that the first spot is also located on its own body (manifestation of a sudden interest towards the hidden spot – which can include turning, or trying to reach the spot with a limb, while viewing in the mirror), then it passes the test.
So far, apart from humans, a few primate species (including chimps and orangutans), dolphins and elephans have passed the test. But this test is not concludent as a statement of consciousness, since many species do not possess stereoscopic vision (rabbits or deers), while other species (such as dogs) have very poor visual resolution, although they are able to recognize their own scent.
A number of experiments conducted at the University of Liverpool have found that sometimes, even humans have difficulties in understanding how mirrors work. For example, after covering a mirror on a wall, participants were asked to guess the points between which they would be able to see their reflection. Surprisingly, many people believe they can see themselves before they are level with the near edge of the mirror. Another question was how large their head would appear on the surface of the mirror, the correct answer being: half of its physical size; yet some participants failed to take into account that a mirror is always halfway between the real person and the image that appears in the mirror, so they estimated it would have a similar dimension to their physical head.
An interesting phenomenon related to mirrors is the so-called “Venus Effect”: a painting of a woman (the goddess Venus, for instance) looking in the mirror, and while we may assume that she is admiring her own face, because we can see her face in the mirror, actually she should be able to see us instead (see the picture above).
[ZME Questions and answers: What is the colour of a mirror?]