Iridologists work from a chart that divides the iris into wedge-shaped zones arranged around the pupil like a clock face, each zone assigned to a specific organ or body system; a typical map places the digestive organs near the center, the extremities near the outer edge, and specific organs, such as the kidneys or lungs, in their own designated sectors, with slight variations between the several competing chart systems in use. An iridology reading examines the color, texture, and any spots, streaks, rings, or other markings within each zone, interpreting these as signs of inherited weakness, current inflammation, or a developing problem in the corresponding organ, well before, practitioners often claim, that problem would show up through conventional testing. The method is generally used as a screening or diagnostic tool pointing toward areas of the body that may need further attention, rather than as a treatment in itself.
Iridology's charts descend from the independent nineteenth century work of Ignaz von Peczely in Hungary and Nils Liljequist in Sweden, and reached wide American use through Bernard Jensen's twentieth century training programs and reference charts, which remain the most commonly taught system today. The central premise, that a fixed map connects each part of the iris to a specific distant organ, has no identified basis in ophthalmology or neuroanatomy: the iris's nerve supply and structure do not correspond to the chart's organ divisions, and no physiological pathway has been proposed, let alone demonstrated, by which a change in a distant organ would produce a specific, localized, readable change in iris tissue.