## Opticks: or, A treatise of the reflections, refractions, inflections and colours of light. ...

**by Sir Isaac Newton**

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*Excerpt:*

DEFIN. IV.

*The Angle of Incidence is that Angle, which the Line described by the incident Ray contains with the 'Perpendicular to the reflecting or re~ /raffing Surface at the 'Point of Incidence. *

DEFIN.V.

*The Angle of Reflection or Refraction, is the Angle which the line described by the reflected or refracted Ray containeth with the Perpendicular to the reflecting or refracting Surface at the Toint of Incidence. *

DEFIN. VI. .

*The Sines of Incidence, Reflexion, and Refraction, are the Sines of the Angles of Incidence _{t} Reflexion, and Refraction. *

DEFIN. VII. .

*The Light whose Rays are all alike Refrangible, I call Simple, Homogeneal and Similiar*; *and that whose Rays are some more Refrangible than others, I call compound, Heterogenal and ^Dissimilar. *The former Light I call Homogeneal, not because I would affirm it so in all respects ; but because the Rays which agree in Refrangibility, agree at least in all those their other Properties which I consider in the following Discourse.

DEFIN. VIII.

*The Colours of Homogeneal Lights, I callTrifnary, Homogeneal and Simple; and those of Heterogenedl Lights, Heterogeneal and Compound. *For these are always compounded of the colours of Homogeneal Lights; as will appear in the following Discourse.

*. *' A X. I. '

*THE Angles of Reflexion, and Refraction, lie in one and the fame 'Plane with the Angle of Incidence. *

Ax. n.

*The Angle of Reflexion is equal to the Angle of Incidence. *

Ax. in.

*If the Refracted Ray be returned diretlly back to the 'Point of Incidence, it shall be refracted into the Line before described by the in* cident Ray. *

A X. IV.

*Refraction out of the rarer Medium intothe denser, is made towards the Perpendicular, that is, so that the Angle of Refraction be less than the Angle of Incidence. *

*AX. *V.

*The Sine of Incidence is either accurately or •very nearly in a given Ratio to the Sine of Refraction. *

Whence if that Proportion be known in any one Inclination of the incident Ray, 'tis known in all the Inclinations, and thereby the Refraction in all cases of Incidence on the fame refracting Body may be determined. Thus if the

B 3 RefraRefraction be made out of Air into Water, the Sine of Incidence of the red Light is to the Sine of its Refraction as 4 to 3. If out of Air into Glass, the Sines are as 17 to 11. In Light of other Colours the Sines have other Proportions: but the difference is so little that it need seldom be considered.

Snppose therefore, that RS *fin Fig. *1.3 represents the Surface of stagnating Water, and that C

the pqint of Incidence in which anyRay coming in the Air from A in the Line A C is reflected or refracted, and I would know whither this Ray shall go after Reflexion or Refraction: I erect lippn the Surface of the Water from the point of Incidence the Perpendicular C P and produce it downwards tp Q, and conclude by the first Axiom, that the Ray after Reflexion and Refraction, shall be found somewhere in the Plane of the Angle of Incidence ACP produced. I let fall tjierefqre upon the Perpendicular CP the Sine of Incidence AD; and if the refle6te4 |lay be desired, I produce AD to B so that D B be equal to A D, and draw C B. For this Line C B sliall be the reflected Ray; the Angle pf Reflexion BCP and its Sine BD being equal to the Angle and Sine of Incidence, as they ought to be by the second Axiom. But if the refracted Ray be desired, I produce AD to H, so that D H may be to AD as the Sine of Refraction to the Sine of Incidence, that is (if the Light be red) as 3 to 4; and about the Center *C *and in the Plane ACP with the RadiusC A describing a Circle ABE I draw Parallel to the Perpendicular CPQ/the Line HE cutting the