The Rainbow Passage is one of the most common standard reading passages used to test an individual's ability to produce connected speech. Like the My Grandfather passage it is used in speech evaluations, but it is also used in studying accents, reading comprehension, as a speech exercise, and for testing language recognition software. There are any number of passages that have been designed to give a good representative sample of English sounds, including Arthur the Rat, My Grandfather, and Comma Gets a Cure. The Rainbow Passage may not be the best, but it is the most famous. My Grandfather seems to be a little more common in the fields of voice and speech therapy, but over all Rainbow seems to be the best known reading passage in the English language.
It was designed to contain almost all the English phonemes (it's missing ʒ and the glottal stop). But it includes some fun things that many other reading passages don't, like the syllabic m in 'prism' and the syllabic l in 'Aristotle'.
The Rainbow Passage
When the sunlight strikes raindrops in the air, they act as a prism and form a rainbow. The rainbow is a division of white light into many beautiful colors. These take the shape of a long round arch, with its path high above, and its two ends apparently beyond the horizon.
There is, according to legend, a boiling pot of gold at one end. People look, but no one ever finds it. When a man looks for something beyond his reach, his friends say he is looking for the pot of gold at the end of the rainbow.
Throughout the centuries people have explained the rainbow in various ways. Some have accepted it as a miracle without physical explanation. To the Hebrews it was a token that there would be no more universal floods. The Greeks used to imagine that it was a sign from the gods to foretell war or heavy rain. The Norsemen considered the rainbow as a bridge over which the gods passed from earth to their home in the sky.
Others have tried to explain the phenomenon physically. Aristotle thought that the rainbow was caused by reflection of the sun's rays by the rain. Since then physicists have found that it is not reflection, but refraction by the raindrops which causes the rainbows.
Many complicated ideas about the rainbow have been formed. The difference in the rainbow depends considerably upon the size of the drops; the width of the colored band increases as the size of the drops increases. The actual primary rainbow observed is said to be the effect of a super-imposition of a number of bows. If the red of the second bow falls upon the green of the first, the result is to give a bow with an abnormally wide yellow band, since red and green light when mixed form yellow. This is a very common type of bow, one showing mainly red and yellow, with little or no green or blue.
While this is a very good reading passage, as such things go, it has been criticized for containing too many difficult vocabulary words. A reading passage isn't any good if the subject can't read it, and words like 'Aristotle' and 'phenomenon' make it hard for children and non-native English speakers to understand and pronounce. It is also very long, so often only the first two paragraphs are used. Many sources don't bother to give the entire passage, but if you are looking for a shorter passage that contains a good number of English phonemes you would do better to use something like My Grandfather rather than cut Rainbow short.
This copy of The Rainbow Passage comes from http://www.uta.fi/FAST/AK11/rainbow.html, which in turn found it in the Voice and Articulation Drillbook by Grant Fairbanks. The Rainbow Passage is pretty well standardized, so this is word for word what you will find in most textbooks.
The Rainbow Passage is a public domain text.