Locke supposes that a person acquainted sensibly with the colours which compose a rainbow, can by the names of such colours in a verbal description, be made visually acquainted with a rainbow. The verbal description will give such a person's intellect a good verbal definition of the word rainbow, but it cannot communicate the sight to the extent that it differs, in any manner from the sights he already knows.

Part II. Of the Extent of Sensible Knowledge. - The Physiology of the Senses: Or, How and what We See, Hear, Taste, Feel and Smell (1856)