The colour of light is determined by the frequency of the ethereal vibrations, as the pitch of sound is determined by the frequency of the aërial ones. The red or purple, for example, of a British maiden’s cheek and lips, the blue, violet, or brown of her eyes, have their strict physical equivalents in the lengths of the waves which issue from them; and these waves are not only as truly mechanical as the waves of the sea, but they are capable of having their mechanical value expressed in numbers. In the work already referred to, a chapter is devoted to the relation which subsists between light and heat and mere mechanical work. In virtue of this relation we can tell the precise amount of work which a given amount of sunshine can perform. Now, the hue of the cheek is caused by the extinction of certain of the solar rays by the colouring matter of the cheek, the residual colour being that seen. Could we interpose the substance to which some English cheeks owe their bloom in the path of a beam passing through a prism, we should probably find the orange and yellow and green of the prismatic spectrum more or less absorbed, the red and a portion of the blue being transmitted. This would give us a purplish blush resembling that of the permanganate of potash, commonly called the mineral chameleon, a solution of which acts upon the spectrum in the manner just described. Inasmuch, then, as we can calculate with perfect exactness the mechanical value of the total light which falls upon the epidermis, a certain fraction of this will express the mechanical value of the cheek’s colour. We do not therefore jest, but speak the words of truth and soberness when we affirm that the rays to which the tinting of any given cheek is due would, if mechanically applied, be competent to move a wheelbarrow through a certain space, or to lift a scuttle of coals to a certain calculable elevation.