Vitamin A was the first of the critical nutritional compounds called vitamins to be discovered. American scientist Elmer V. McCollum identified the compound in his experiments with the nutrition of rats. He found that the rats grew and remained healthy when fed a diet of butter and egg yolk. But if lard or olive oil was substituted for the butter, the rats declined in health and eventually died. Believing that some essential substance was contained in the butter, he extracted a yellow-colored oil from the butter which he called "fat-soluble A", now called vitamin A. Vitamin A or retinol is an alcohol with composition C20H29OH.
Vitamin A is found in eggs, milk, butter, and green and yellow vegetables. It increases resistance to infection and promotes good night vision (Shipman and Wilson).
Shipman and Wilson