Maupertuis' Three Laws of Motion for Light and Action as a Universal Principle

  In his Accord de différentes lois de la nature of 1744 Maupertuis gives Three Laws for the motion of light.

"Here are the laws that light follows.

The first is that, in a uniform medium, it moves in a straight line.

The second, that when light meets a body which it cannot penetrate, it is reflected; and the angle of its reflection is equal to the angle of its incidence; that is to say, after its reflection, it makes an angle with the surface of the body equal to that under which it had met it.

The third is, that when the light passes from one diaphanous medium to another, its path after the meeting of the new medium, makes an angle with that which it held in the first; & the sine of the angle of refraction is always in the same ratio to the sine of the angle of incidence. If, for example, a ray of light passing from the air into the water is broken so that the sine of the angle of its refraction is three quarters of the sine of its angle of incidence; under some other obliquity that it meets the surface of the water, the sine of its refraction will always be three quarters of the sine of its new incidence."

He also contrasts the term, quantity of action, with Descartes' quantity of motion in Letter X found in his Works and describes action as a universal principle governing all motion.

"There is a truly universal principle, from which these Laws follow, in regards to the movement of hard bodies, elastic bodies, light, and all bodily substances: That is, in all the changes which occur in the Universe, the sum of the products of each body multiplied by the space which it traverses, and by the speed with which it traverses it, what is called the quantity of action, is always the smallest that is possible."