Dr. Ignatz von Peczely came up with the idea for Iridology when he noticed a black streak through the eye of an
owl who had broken its leg.
The theory behind Iridology suggests that the condition of the body can be determined by examining a patient's eyes and observing various areas of each iris. Iridology proponents suggest that nerve impulses carried to the eye through the optic nerve (and, in turn, carried to the optic nerve by the central nervous system) are visibly represented in the Iris.
Iridologists have developed Iris Charts which map regions in the iris of the left and right eyes to various parts of the body. A disturbance (discoloration, etc.) that may be visible in various parts of the iris indicate a weakness or problem in some corresponding area of the body.
One of the problems with Iridology is that there is no uniform iris chart. In fact, over 20 are known to exist, and even though they are largely similar, they are not consistent.
The iris chart developed by Dr. Bernard Jensen, one of the leading practitioners of Iridology, is one example. In his chart, organs in the left side of the body are represented in the left iris, and organs in the right side of the body are represented in the right iris. In Dr. Jensen's chart, a ring is formed around the pupil by an area of the iris that represents the assorted parts of the human digestive system. A thin ring around the outermost area of the iris represents the skin. The area in between these two rings is divided, more or less radially, into segments. Each segment represents a different organ. Around the 12 o'clock position different areas of the brain / psyche are represented. Around the 6 o'clock position, the legs and feet.
One of Dr. Jensen's more interesting theories is the relationship between problems in the colon and problems in other areas of the body (a topic which deserves its own writeup, so I'll be brief). As the digestive track wraps around the inner ring of the iris in his iris chart, as explained above, each organ corresponds to a particular segment of the digestive tract. In fact, Dr. Jensen goes so far as to suggest that a problem with a particular area of the digiestive track can cause problems in the organ that corresponds to that part of the colon in his Iris chart. This is due to autonomic nerve response.
Despite the fact that the explanations behind Iridology are very elaborate, and that it would be extreminly useful if the iris really was a readout of the body's condition, most people in the medical science community are dismissive of it. In fact, several studies have been performed, by UC San Diego, and others. In these studies, leading Iridologists, including Dr. Jensen, were shown photographs of patient's eyes, many of whom had assorted ailments, to see if they could determine which patients were healthy and which had problems, and in what areas of their body. The result of these studies suggest that diagnosis performed through Iridology is no more statistically accurate than guessing.