M. Comte's contempt for the speculations of the undulationists seems to have prevented his acquainting himself with their reasonings, and even with the laws of phenomena on which they have reasoned, although these form by far the most striking and beautiful addition which Science has received in modern times. He adduces, as an insuperable objection to the undulatory theory, a difficulty which is fully removed by calculation in every work on the subject:--the existence of shadow[256]. He barely mentions the subject of diffraction, and Young's law of interferences;--speaks of Fresnel as having applied this principle to the phenomena of coloured rings, "on which the ingenious labours of Newton left much to desire;" as if Fresnel's labours on this subject had been the supplement of those of Newton: and after regretting that "this principle of interferences has not yet been distinctly disentangled from chemical conceptions on the nature of light," concludes his chapter. He does not even mention the phenomena of dipolarization, of circular and elliptical polarization, or of the optical properties of crystals; discoveries of laws of phenomena quite as remarkable as any which can be mentioned. M. Comte's favourite example of physical research is Thermotics, and especially Fourier's researches with regard to heat. It is shown[257] in the History of Thermotics, that the general phenomena of radiation required the assumption of a fluid to express them; as appears in the _theory of exchanges_[258]. And the explanation of the principal laws of radiation, which Fourier gives, depends upon the conception of material molecular radiation. The _flux_ of caloric, of which Fourier speaks, cannot be conceived otherwise than as implying a material flow. M. Comte apologizes[259] for this expression, as too figurative, and says that it merely indicates a _fact_. But what is the flow of a current of fluid except a fact? And is it not evident that without such expressions, and the ideas corresponding to them, Fourier could neither have conveyed nor conceived his theory?