produced by the heat as a slight degree of dimness: hence a bright yellow must immediately appear; this, as the dimness increases, must still appear deeper, more condensed, and redder, and at last pure and ruby-red. The colour has now reached the extreme point of depth, and if we suppose the same degree of semi-opacity still to continue, the dimness would now spread itself over a dark ground, first producing a violet, then a dark-blue, and at last a light-blue, and thus complete the series of the appearances.
We will not assert that this mode of explanation will suffice in all cases; our object is rather to point out the road by which the all-comprehensive formula, the very key of the enigma, may be at last discovered.—Note S.