There are few of the applications of optical science so universally interesting as that which has for its object the explanation of the colours of natural bodies. Sir Isaac Newton was the first person who ventured to refer to one general principle all the variety of colours which are found in nature; and he has maintained his opinions on this subject with a confidence in their accuracy which seems to have confounded his adversaries: For while his analysis of light, the most perfect of all his labours, exposed him to the most harassing controversies, his theory of natural colours, the least perfect of his speculations, was allowed to pass without examination or censure.