Theoretical physics, from the very beginning, has been a synthesis of phenomena in terms of mechanics; that is, in terms of substance and motion. For example, the physical properties of heat, sound, and light are expressed by the same mathematical formulæ which express the motion of a wave in water. To distinguish between them, we assign different names to the substances involved, as a molecule of air, a corpuscle of light, or an æther. But whatever names we may give to them, or however we may try to distinguish them, we assign to all of them the common attribute of mass, or inertia, which is the only essential coefficient in a mechanical equation. And we use for all these different phenomena the same formula of motion, the quotient of the distance by the time. These quantities, mass and motion, when combined give us the law of mechanical energy, and our synthesis rests on the single fact that heat, sound, and light may be changed into mechanical motion, and may be produced by it. This energy is then their common and mutable factor. But the objective phenomena of heat, light, and sound are cognisable to us through three separate sense-organs and are perceived as temperature, sight, and tone. These sensations are fundamentally different and, in fact, to confuse any of them with another is one of the surest indications of insanity. Thus the world as depicted by the physicist does not correspond with our world of sensation; nor does he attempt to do more than to discuss a restricted set of attributes, and not even those which really distinguish heat, light, etc., as such. Newton, in his earliest published work, made evident this essential difference between the fields of physics and psychology. By means of a prism he refracted a beam of white sunlight into a continuous spectrum. He then placed a screen, containing a narrow slit, behind the prism in such a way as to permit only a very thin ray from the spectrum to pass on, and through, a second prism fastened parallel to the first prism. (I call it a homogeneous light ray to distinguish it from its psychological analogue, colour.) No matter what portion of the spectrum was used for the second prism, there was no further change of colour; the ray merely suffered a second angular deviation equal to that produced by the first refraction. He also recombined the whole spectrum by a reversed prism and obtained a single ray parallel to the original ray and pure white in colour. As a result of his experiments, he announced the following law: a primary ray of light is one which has a definite and specific angle of refraction by a prism, and each such primary ray is, to the eye, a primary colour. White is therefore a mixture of an indefinitely large number of primary rays, each possessing a different angle of deviation when passed through a prism; and when so separated each primary ray is seen by the eye as a primary colour.