That Light spreads spherically, almost as Sound does. Whether Light takes time to spread. Experience seeming to prove that it passes instantaneously. Experience proving that it takes time. How much its speed is greater than that of Sound. In what the emission of Light differs from that of Sound. That it is not the same medium which serves for Light and Sound. How Sound is propagated. How Light is propagated. Detailed Remarks on the propagation of Light. Why Rays are propagated only in straight lines. How Light coming in different directions can cross itself. CHAPTER II. On Reflexion. Demonstration of equality of angles of incidence and reflexion. Why the incident and reflected rays are in the same plane perpendicular to the reflecting surface. That it is not needful for the reflecting surface to be perfectly flat to attain equality of the angles of incidence and reflexion. CHAPTER III. On Refraction. That bodies may be transparent without any substance passing through them. Proof that the ethereal matter passes through transparent bodies. How this matter passing through can render them transparent. That the most solid bodies in appearance are of a very loose texture. That Light spreads more slowly in water and in glass than in air. Third hypothesis to explain transparency, and the retardation which Light suffers. On that which makes bodies opaque. Demonstration why Refraction obeys the known proportion of Sines. Why the incident and refracted Rays produce one another reciprocally. Why Reflexion within a triangular glass prism is suddenly augmented when the Light can no longer penetrate. That bodies which cause greater Refraction also cause stronger Reflexion. Demonstration of the Theorem of Mr. Fermat. CHAPTER IV. On the Refraction of the Air. That the emanations of Light in the air are not spherical. How consequently some objects appear higher than they are. How the Sun may appear on the Horizon before he has risen. That the rays of light become curved in the Air of the Atmosphere, and what effects this produces. CHAPTER V. On the Strange Refraction of Iceland Crystal. That this Crystal grows also in other countries. Who first-wrote about it. Description of Iceland Crystal; its substance, shape, and properties. That it has two different Refractions. That the ray perpendicular to the surface suffers refraction, and that some rays inclined to the surface pass without suffering refraction. Observation of the refractions in this Crystal. That there is a Regular and an Irregular Refraction. The way of measuring the two Refractions of Iceland Crystal. Remarkable properties of the Irregular Refraction. Hypothesis to explain the double Refraction. That Rock Crystal has also a double Refraction.