Human iris. Source: Michael Reeve, CC3.0 license
Iridology is the diagnosis of disease based on the appearance of the iris, the colored part of the eye. The practice is considered an "alternative" medicine in that it is not an accepted practice of the basic medical establishment. The reason for it's lack of acceptance is a lack of evidence that it works, which is easily understood if the role the iris plays in the body is known. These are also the reasons why it is not science.
The iris is a pigmented muscle that controls the size of the pupil, the entry point of the eye for light. The size of the pupil determines how much light enters the eye. The iris does not take part in disease and is a very small tissue to observe. These facts alone would make the practice extremely complex, if it was even successful. Actually, many iridologists now use computer programs to determine patterns because visual inspection of the eye is so difficult, though this is what the practice is based upon.
Iridology is believed to have been developed by a Hungarian physician in the 1840s. Over the years, the practice has been tested several times with no success in verification. Iridologists have failed to predict disease when presented with cases to confirm the field, and no controlled scientific studies have been done on the iris to determine what portions are associated with what disorders. Many charts used by iridologists were developed roughly 70 years ago with no scientific basis. The charts only indicate body area, and do not aid in indicating the exact disease or disorder, which would be necessary if the process worked. Physicians point out that many of the diseases attributed to iris appearance are questionable themselves, being debated syndromes that may not even exist, whereas known and serious conditions are not recognized. This ambiguity of iridology is due to the lack of information provided by the iris and the amount of guessing that is inherent in the practice. The less specific the guess, the greater chance of being somewhat correct over time.
According to QuackWatch, a site dedicated to uncovering pseudoscience and medical malpractice, both the earliest current technological era studies and recent studies have not cast iridology in a pleasant light. In 1979, three iridologists consulted 143 patients and were unsuccessful in discriminating between those who were healthy and those with kidney disease, misdiagnosing 88% of the healthy individuals with a disease they did not have. In 1980, an Australian iridologist and five Dutch iridologists were also tested, failing in a similar manner. It has also been shown repeatedly that cancers are missed or misdiagnosed when using this practice.
Medicine is based on science - an objective process by which observations are confirmed by experimentation while controlling for variables. In order for iridology to be a real scientific field of study that contributes to medical diagnosis, controlled experimentation would have to show that the purported visualization of the iris directly correlates with disease occurrence. In fact, the opposite has been shown. The rates of diagnosis are not even at par with what would be expected of random guessing - only 3 of 68 cancer cases were properly diagnosed in a 2005 study.
Flecks, streaks, and discolorations of the eye are indicative of eye problems - Research has repeatedly shown that the iris does not shed light on anatomical disorders. The practice is much like the humors approach of medical science centuries ago before anatomical exploration showed the causes of disease, but we are more advanced now, and iridology harkens back to 18th century quackery. Iridology is not science.